Look at the SEC right now. #3 Alabama, #5 Auburn, #6 Texas A&M, #8 LSU, and #10 Ole Miss are all in the same DIVISION. There will be controversy if 2 of those 4 are not selected, especially if....when...Florida St., Oklahoma, or Oregon lose a game. There will also be controversy if the ACC, Big 12, or PAC 12 champion is left out.
8 teams is a nice number because the math works for a 3-round tournament and you can make sure to get the four BEST and the four MOST DESERVING in the top-8. Though, some of the "best" and "most deserving" will overlap.
While it would make the conference championship games more exciting, I hope they don't give the Autonomy 5 champions autobids. Big Ten champ Wisconsin was UNranked a couple of years ago. The ACC and Big Ten champs are often outside the top-10.
I would say that ANY conference champion ranked in the final top-10 gets an autobid, as long as there is room (ie, if the top-8 aren't all conference champions already). That would help to reward conference champions and actually enhance the regular season. If the 8-team playoff has 5 SEC teams, the importance of the SEC regular season and championship is SEVERELY compromised.
I personally like the 10-team format where you have a play-in round. You give a conference champion in the top-10 an auto bid to the top-6 seeds, as long as there is room.
Then, borrowing from the NCAA tournament, you have an 8-seed play-in game between the top two conference champions that aren't in the top-6 (usually, outside the top-10)...and then a 7-seed game between the last two at large contenders that aren't in the top-6 (whether knocked out by a conference champion or just not ranked in the top-6).
This allows more conference champions access to the CFP, but without overcrowding the field. It also forces at large teams to duke it out before you even get to the quarterfinals, which helps to protect the significance of the regular season (ie, favoring conference champs).