There isn't a true magical number that the P5 has to get to.....it doesn't need to be 64 or 60 or anything like that.
Within the PAC, ACC, B1G, SEC there are 53 schools. If they pull Texas, TT, Oklahoma, OSU and Kansas it that will be 58 schools.
The B12 can then go out and add New Mexico, Tulane, Cincinnati, Memphis, Houston, Colorado St, BYU and get to 12. They would more or less be a true tweener conference with a TV deal approaching 1/2 the value of the P5 like the old BE football conference did compared to the other BCS conferences.
The AAC then can go for UTSA, Rice, Ohio, Army to stick with their expansion themes and become more of an East Coast based conference with UConn, Army (2 hours from the East Coast), Temple, Ohio (5 hours from the East Coast), Navy, ECU, USF, UCF. Rice for academics, UTSA to backfill for Houston. It would not be as good of a conference as the B12.
The MWC brings in NMSU and UTEP. UTEP by then will only have North Texas in CUSA so would 100% welcome a move.
The MAC brings Marshall back in as a replacement for Ohio which they agree to because of the easy travel.
CUSA down to 9 schools but only 3 in the West adds ULL, AState and Texas St to go with UNT, La Tech and So Miss. CUSA then would be at a nice even 12.
SBC down to 7/6 all sport. Liberty and EKU are the only ones still interested in the SBC all sports. UMass FB only to get the conference back to 10 FB members.
Then you have a P4 with 58 schools in it and then a G6 with 72 schools. The G6 force a 2nd autobid and playoff expansion to 8 teams/8 NYW games.