3.
Not because there shouldn't be more, but because the committee will opt to give teams with fewer losses against weak competition many of the spots in the name of "fairness". For instance, this year's Liberty team; their 4 toughest opponents (before bowl game) and their Sagarin ratings:
#82 Jacksonville State 9-4
#90 Bowling Green 7-6
#96 Western Kentucky 8-5
#98 New Mexico State 10-5
In comparison, we played 7 teams in the top 50. And Virginia was our 9th toughest opponent at #91, which would've made them Liberty's 3rd toughest opponent. Most of the SEC would've been 13-0 with Liberty's schedule.
I guarantee you that if the 12 team playoff were this year, Liberty would've gotten in.