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. For more information, see backup_dev_type.

Syntax: rman_filesperset = <number>

Default: 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