Should be 12 but with more than 6 auto bids.Should be 8 teams. One each from the P5, one from a non-P5 (which includes ND), and two at large to fill in the eight slots. Also, no conference can have more than 2 teams.
Bring back BCS formula auto bid for any conference champ in the top 20.
2+ at larges to fill in the rest of the gaps.
That would be the way you could expand the playoff. Leave open the possibility of 2 or 3 bids for the big conferences, but also INCREASE the value of regular season games