Feel if they lose just Miami and Notre Dame to the Big Ten and Florida State, North Carolina, Virginia Tech, and Clemson to the SEC, they’d be able to get away with only adding
UConn from the Big East/FBS Independent
Tulane, South Florida, and Memphis from the American Conference
and be a 16-member conference.
However, if they lose everyone except Boston College and Wake Forest (unsure who all goes where), I feel they’d add
UConn from the Big East/FBS Independent
Tulane, South Florida, Memphis, UTSA, and Rice from the American Conference
Villanova from the Big East as a non-football member
Army and Navy from the American as football-only members
Air Force from the Mountain West
Oregon State, Washington State, Boise State, Colorado State, and San Diego State from the Pac-12
which would give them amongst the best they can get from the Group of Six (better to prioritize value over geography), as well as allow them to have 15 full member schools (see below paragraph for significance of this); would be a 16-member conference in football and a 15-member conference in non-football.
If I remember correctly, as long as they never officially dip below 15 full members (aka if in the year the schools leave, as long as who they add would still leave the ACC with 15 full member schools with who they add, they’re good), they retain their TV deals; otherwise they can lose them. As shown by the Pac-12 it would be a HUGE mistake to lose a TV deal with ESPN (its why SMU is in the ACC; originally they were going to join Pac-12 with San Diego State when they had only lost USC and UCLA). Both lineups satisfy this.
Personally feel they’d still be a Power Four either way as long as the additions all join in the same season they lose the schools. Put both through online realignment simulators and they agreed.