I think an 8-team playoff makes a lot of sense. An issue with the BCS was there were teams ranked #3 and #4 that may have had a legit resume to be considered to play for a national title. To solve that, the 4-team playoff allows teams #1-#4 to battle it out to see who the true champion is.
My point is that with 5 power conference champions (although power is a loose term for some of these conference...looking at you AAC), and incredible depth in certain conferences, there is definitely 8 quality teams that could win 3 playoff games. The #5 or #6 ranked team in the country, at years end, typically isn't that far away from #3-#4 in terms of their resume and quality of team. I do think that the teams ranked between #9-#12, for example, usually are nowhere near the caliber of team to actually win a national championship, making anything more than an 8-team playoff just too large.
I'm not taking into consideration any logistics of playing 3 playoff games or its affect on student-athelete schedules or any other relevant factors. From a competition standpoint only, I think 8 teams would be a perfect number for a playoff.