This parameter defines the number of database files stored in a save set by RMAN with an external backup library, that is, backup_dev_type = rman_util|rman_disk|rman_stage.
rman_filesperset = <number>
Default value: 0, which means one file per save set for non-incremental backups or all files in one save set for incremental backups
See also:
RMAN Backup with an External Backup Library
Oracle documentation