It’s Six. That number allows the right number of teams a chance. 2014 had a tight and obvious six. 2015 had seven contenders (fraud-ish Iowa probably gets in, leaving Ohio State out). 2016 has a tight and obvious six.
Six also best preserves the regular season. Seeding would matter. A LOT. #1 and #2 getting a bye would be a massive advantage, both on players’ bodies and for scouting. #3 and #4 must play a quarterfinal, yet would have a home crowd behind them. #5 and #6 get a ticket to the party, but face the daunting task of a road game against a Top Four opponent.
#7 gripes, but in most years a team outside the Top Six would either have lost multiple games, played a very soft schedule, not won their conference, or all of the above.