6 teams would be good if you take the conference champs from the power 5 and the highest ranked non power 5 conference champ (Boise State this year). I don't like having an at large team that's not a conference champ. But then with the top 2 teams getting a bye, there'd be a big debate every year on who is #2 vs #3, which would be a big advantage to not have to play the first round game.
8 teams makes the most sense, but then you have non conference champ at large teams. Would be easier if the ACC just went away and stay with 4 team playoff of conference champs.