Ace is a one-back formation usually one back, two TE (one TE, one H-Back) and two receivers (or one back, one TE, three receivers). Provides more gaps that a defense has to account for and they also have to account for the TEs in the passing game. 12 is just the personnel grouping for an Ace formation. A two back formation would be a 21 personnel package. 11 person would be the one-back, one TE, three receivers. 10 personnel would be one back, no tight ends (four wide). 0 personnel is 5 wide and so on and so on.