I think that  3 parameters every Oracle DBA should know and keep the sintaxis in mind are the following:

1. Maximum Size of Backup sets.
2. Maximum Size of Backup Pieces.
3. Setting How many datafiles per Backup Piece.

These parameters look very common in our daily life however how many DBA's keep the sintaxis and common errors in mind? if you are a new DBA and you don't know what these options are for you can see these examples as your introduction to the Oracle DBA life, in the other hand if you are an Expert this is an opportunity to review your concepts and save the sintaxis in your mind:

Filesperset: This parameter set the number of datafiles per Backup Piece.

RMAN> backup database filesperset=1;

Starting backup at 25-DEC-14
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00001 name=+DATA/orcl/datafile/system.437.865029795
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3ttcw_.bkp tag=TAG20141225T172737 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00002 name=+DATA/orcl/datafile/sysaux.353.865029795
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3txmd_.bkp tag=TAG20141225T172737 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00003 name=+DATA/orcl/datafile/undotbs1.354.865029795
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v0r2_.bkp tag=TAG20141225T172737 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00004 name=+DATA/orcl/datafile/users.256.865029795
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v1v9_.bkp tag=TAG20141225T172737 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=+DATA/orcl/datafile/tbslocal.440.865852271
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v32g_.bkp tag=TAG20141225T172737 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00008 name=+DATA/orcl/datafile/fdatbs.449.867242657
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v450_.bkp tag=TAG20141225T172737 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00005 name=/home/oracle/mydb.dbf
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v58w_.bkp tag=TAG20141225T172737 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00007 name=+DATA/orcl/datafile/tbslocal.259.865852271
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v6d7_.bkp tag=TAG20141225T172737 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 25-DEC-14

Starting Control File and SPFILE Autobackup at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/autobackup/2014_12_25/o1_mf_s_867259670_b9s3v71k_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 25-DEC-14

Result:

RMAN> list backup of database;

List of Backup Sets
===================

BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
104 Full 272.52M DISK 00:00:02 25-DEC-14
BP Key: 104 Status: AVAILABLE Compressed: NO Tag: TAG20141225T172737
Piece Name: /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3ttcw_.bkp
List of Datafiles in backup set 104
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
1 Full 2352094 25-DEC-14 +DATA/orcl/datafile/system.437.865029795

BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
105 Full 286.28M DISK 00:00:03 25-DEC-14
BP Key: 105 Status: AVAILABLE Compressed: NO Tag: TAG20141225T172737
Piece Name: /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3txmd_.bkp
List of Datafiles in backup set 105
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
2 Full 2352097 25-DEC-14 +DATA/orcl/datafile/sysaux.353.865029795

BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
106 Full 2.81M DISK 00:00:00 25-DEC-14
BP Key: 106 Status: AVAILABLE Compressed: NO Tag: TAG20141225T172737
Piece Name: /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v0r2_.bkp
List of Datafiles in backup set 106
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
3 Full 2352100 25-DEC-14 +DATA/orcl/datafile/undotbs1.354.865029795

BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
107 Full 11.78M DISK 00:00:00 25-DEC-14
BP Key: 107 Status: AVAILABLE Compressed: NO Tag: TAG20141225T172737
Piece Name: /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v1v9_.bkp
List of Datafiles in backup set 107
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
4 Full 2352103 25-DEC-14 +DATA/orcl/datafile/users.256.865029795

BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
108 Full 1.03M DISK 00:00:01 25-DEC-14
BP Key: 108 Status: AVAILABLE Compressed: NO Tag: TAG20141225T172737
Piece Name: /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v32g_.bkp
List of Datafiles in backup set 108
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
6 Full 2337190 25-DEC-14 +DATA/orcl/datafile/tbslocal.440.865852271

BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
109 Full 1.03M DISK 00:00:01 25-DEC-14
BP Key: 109 Status: AVAILABLE Compressed: NO Tag: TAG20141225T172737
Piece Name: /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v450_.bkp
List of Datafiles in backup set 109
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
8 Full 2339703 25-DEC-14 +DATA/orcl/datafile/fdatbs.449.867242657

BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
110 Full 1.03M DISK 00:00:01 25-DEC-14
BP Key: 110 Status: AVAILABLE Compressed: NO Tag: TAG20141225T172737
Piece Name: /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v58w_.bkp
List of Datafiles in backup set 110
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
5 Full 2352106 25-DEC-14 /home/oracle/mydb.dbf

BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ ---------------
111 Full 1.03M DISK 00:00:01 25-DEC-14
BP Key: 111 Status: AVAILABLE Compressed: NO Tag: TAG20141225T172737
Piece Name: /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T172737_b9s3v6d7_.bkp
List of Datafiles in backup set 111
File LV Type Ckp SCN Ckp Time Name
---- -- ---- ---------- --------- ----
7 Full 2337190 25-DEC-14 +DATA/orcl/datafile/tbslocal.259.865852271

As you can see each Backup Piece has only one Datafile inside.

MAXPIECESIZE: This option can't be specified in the clause "BACKUP", this must be specified in the clause "CONFIGURE". With this option you can set the Maximun Size of the Backup Piece. Note that RMAN will try to follow your value however it could be that RMAN create the Backup Piece with a size a little bit bigger or a little bit smaller. The value of MAXPIECESIZE must be between 1 Kb and 2048 Gb. In the following example I will show you how to set the maximum size to 50M and then we will confirm this with "du -sh":

RMAN> CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE 50M;

old RMAN configuration parameters:
CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE 2 M;
new RMAN configuration parameters:
CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE 50 M;
new RMAN configuration parameters are successfully stored

Once you have set this parameter you can perform a backup as usual: 

RMAN> backup database;

Starting backup at 25-DEC-14
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=22 device type=DISK
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00001 name=+DATA/orcl/datafile/system.437.865029795
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sopr_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: starting piece 2 at 25-DEC-14
channel ORA_DISK_1: finished piece 2 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sp4q_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: starting piece 3 at 25-DEC-14
channel ORA_DISK_1: finished piece 3 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sq50_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: starting piece 4 at 25-DEC-14
channel ORA_DISK_1: finished piece 4 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sr56_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: starting piece 5 at 25-DEC-14
channel ORA_DISK_1: finished piece 5 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6ss5o_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: starting piece 6 at 25-DEC-14
channel ORA_DISK_1: finished piece 6 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6st5w_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:06
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00002 name=+DATA/orcl/datafile/sysaux.353.865029795
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6svwd_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: starting piece 2 at 25-DEC-14
channel ORA_DISK_1: finished piece 2 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6swb3_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: starting piece 3 at 25-DEC-14
channel ORA_DISK_1: finished piece 3 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sxbh_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: starting piece 4 at 25-DEC-14
channel ORA_DISK_1: finished piece 4 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sybr_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: starting piece 5 at 25-DEC-14
channel ORA_DISK_1: finished piece 5 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6szc2_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: starting piece 6 at 25-DEC-14
channel ORA_DISK_1: finished piece 6 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6t0cs_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:06
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00005 name=/home/oracle/mydb.dbf
input datafile file number=00004 name=+DATA/orcl/datafile/users.256.865029795
input datafile file number=00007 name=+DATA/orcl/datafile/tbslocal.259.865852271
input datafile file number=00003 name=+DATA/orcl/datafile/undotbs1.354.865029795
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6t279_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00006 name=+DATA/orcl/datafile/tbslocal.440.865852271
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6t38t_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00008 name=+DATA/orcl/datafile/fdatbs.449.867242657
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6t4d2_.bkp tag=TAG20141225T181812 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 25-DEC-14

Starting Control File and SPFILE Autobackup at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/autobackup/2014_12_25/o1_mf_s_867262708_b9s6t545_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 25-DEC-14

Now let's confirm the size:

[oracle@a1 ~]$ du -sh /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/*.bkp
51M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sopr_.bkp
51M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sp4q_.bkp
51M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sq50_.bkp
51M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sr56_.bkp
51M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6ss5o_.bkp
23M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6st5w_.bkp
51M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6svwd_.bkp
51M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6swb3_.bkp
51M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sxbh_.bkp
51M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6sybr_.bkp
51M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6szc2_.bkp
37M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6t0cs_.bkp
17M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6t279_.bkp
1.1M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6t38t_.bkp
1.1M /u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T181812_b9s6t4d2_.bkp
[oracle@a1 ~]$

As you can see the maximum size was set to 51M (a little bit bigger). 

MAXSETSIZE: This parameter set the Maximum Size of every BackupSet. The value of MAXSETSIZE must be between 1 Kb and 2048 Gb.

Note: If you specify a size smaller than the smallest datafile in your database you will get the following error:

RMAN> backup database maxsetsize 10M;

Starting backup at 25-DEC-14
using channel ORA_DISK_1
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of backup command at 12/25/2014 17:52:03
RMAN-06183: datafile or datafile copy /home/oracle/mydb.dbf (file number 5) larger than MAXSETSIZE

Example:

RMAN> backup database maxsetsize 2G;

Starting backup at 25-DEC-14
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00005 name=/home/oracle/mydb.dbf
input datafile file number=00001 name=+DATA/orcl/datafile/system.437.865029795
input datafile file number=00002 name=+DATA/orcl/datafile/sysaux.353.865029795
input datafile file number=00003 name=+DATA/orcl/datafile/undotbs1.354.865029795
input datafile file number=00007 name=+DATA/orcl/datafile/tbslocal.259.865852271
input datafile file number=00004 name=+DATA/orcl/datafile/users.256.865029795
input datafile file number=00006 name=+DATA/orcl/datafile/tbslocal.440.865852271
input datafile file number=00008 name=+DATA/orcl/datafile/fdatbs.449.867242657
channel ORA_DISK_1: starting piece 1 at 25-DEC-14
channel ORA_DISK_1: finished piece 1 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7d0st_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 2 at 25-DEC-14
channel ORA_DISK_1: finished piece 2 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7d35p_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 3 at 25-DEC-14
channel ORA_DISK_1: finished piece 3 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7d45y_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 4 at 25-DEC-14
channel ORA_DISK_1: finished piece 4 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7d565_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 5 at 25-DEC-14
channel ORA_DISK_1: finished piece 5 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7d66c_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 6 at 25-DEC-14
channel ORA_DISK_1: finished piece 6 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7d76m_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 7 at 25-DEC-14
channel ORA_DISK_1: finished piece 7 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7d86x_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 8 at 25-DEC-14
channel ORA_DISK_1: finished piece 8 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7d973_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 9 at 25-DEC-14
channel ORA_DISK_1: finished piece 9 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7db79_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 10 at 25-DEC-14
channel ORA_DISK_1: finished piece 10 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7dc7q_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 11 at 25-DEC-14
channel ORA_DISK_1: finished piece 11 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7dd7z_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: starting piece 12 at 25-DEC-14
channel ORA_DISK_1: finished piece 12 at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/backupset/2014_12_25/o1_mf_nnndf_TAG20141225T182759_b9s7df84_.bkp tag=TAG20141225T182759 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:14
Finished backup at 25-DEC-14

Starting Control File and SPFILE Autobackup at 25-DEC-14
piece handle=/u01/app/oracle/flash_recovery_area/ORCL/autobackup/2014_12_25/o1_mf_s_867263294_b9s7dgkv_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 25-DEC-14

Result:

Since my database is small RMAN didn't create more Backupsets, my database fits in only 1 Backupset:

RMAN> list backup of database summary;

List of Backups
===============
Key TY LV S Device Type Completion Time #Pieces #Copies Compressed Tag
------- -- -- - ----------- --------------- ------- ------- ---------- ---
143 B F A DISK 25-DEC-14 12 1 NO TAG20141225T182759