I think 8 would certainly be enough to come up with a fair champion. But IB is right too, people still complain about who doesn't get in the field of 64, or however many it is. Our own coach wants it to go to 128.
I had this thought before and people disagreed with me, but I think if you go to an 8 team playoff, you destroy all the bowls, or at the very least, you NIT them. Now, I know there are way too many bowls as it is and is the GoDaddy Bowl really an event anyway? No. But I'm talking about the Outback, Gator, Capital One, Cotton, Holiday level bowls, which are still events. I think they eventually get wiped out. Because you've created the new, more inclusive event, so the fans (and even players probably) of those not included just aren't going to be as interested in those bowls.
Maybe that's ok too, I don't know. But if I were a betting man, I'd put my money on that being what happens. Not sure they can have 7 bowls host the 7 games either, the travel money for those fans who make the finals could be crazy. Which means some of the playoffs would have to be held at the higher seeds sites.
These games are NIT now. Hell the Cotton Bowl, Orange Bowl and Sugar Bowl were NITesque bowl games this year.
Have all the NIT bowls first, get them out of the way and then start the "playoff". 4 is better than 2. At least it doubles the field. 8 would be better.