The minimum of commons you can have (and still win games) is 8. You need to have 7 of your best cards, 3 commons as subs and 5 commons on the bench. Coz you gotta be sure that even if all 3 commons where selected in a game, the other 5 would be good cards, able to win the matchups.
Though if you played with 7 good cards + 8 commons, you'd have to energize all the time.