If you look at the current bylaws and see the 2 options for a conference
to hold a CCG, then you will discover that a 15 member conference is temporary solution (probably this year only).
Option 1 (12 or more teams)
2 divisions and each division plays a round robin schedule.
A division with 8 and a division with 7 can't play a round robin division schedule AND play the same number of total conference games because the math doesn't work for the cross-divisional games. If you doubt me, then check out the MAC's conference schedule from 2011. Bowling Green doesn't play divisional foe Akron and Kent State doesn't play divisional foe Buffalo that season. When the MAC had 13 members, it was in direct violation of the bylaws. NCAA and the other conferences were willing to overlook the MAC because they are small potatoes. Can you say the same thing about a P5 conference?
Option 2 (10 members)
Play a round robin conference schedule. If the ACC with 15 members decides to abandon the divisions, then it would have to play a round robin conference schedule or 14 games. Currently, the bylaws state a team can only play 12 regular season games or 13 (if 1 of the games is @Hawai'i). Option 2 isn't really an option for the ACC.
ACC decides to no longer play a CCG. There is nothing in the bylaws that state a conference with 12 or more members is required to play a CCG. The ACC with 15 members could scrap the divisions, play 8-10 conference games, and not play a CCG.