If you don't have any rollback segs specified in your init.ora, Oracle will calculate the number of RB segments it needs based on the following formula: number_of_RB_to_put_online = TRANSACTIONS/
TRANSACTIONS_PER_ROLLBACK_SEGMENT. If you have this number of RB segments created it will bring all online. If you have more than this number of RB segments it will left the others in offline mode.