Hard to say. It depends on what P4 requires. Remember, we're talking about professional football now. There are some schools that simply cannot compete in football at highest level. In reality, there are more P4 schools than NFL teams and far fewer capable of winning the championship. They are all either in the Big Ten or SEC. There could be a professional league of affiliated universities as small as 30 teams. 20 teams would probably be enough and that's probably ten too many.
There could be a situation where teams like Cal, Stanford, Northern, Duke ,Wake Forest, etc wouldn't mind playing in a lower division if they could maintain their conference affiliation for other sports at the highest level.
Not every FCS team would want to compete against even MAC level schools, but some could compete very well.
What I'm certain of is nobody really knows what to future holds. Does anyone know a television executive what is going to happen? They are the ones controlling things.