Well, it's not like UNC/UVA/NCSU/VT are traditional powers in CFB. So it's not like the ACC/XII losing them would necessarily upset the apple cart when it comes to football. It'd be a major loss for basketball, but given that the SEC isn't the most tremendous basketball conference in the world, it might be an appropriate rebalancing...
FSU/ND would be losses for the ACC/XII (albeit "loss" considering ND isn't really applicable since they're not fully in the ACC now).
But let's say, sure, you lose them all... Who would be worth it to get the ACC/XII to 20?
Traditional ACC schools you'd keep:
Clemson
Duke
GaTech
Miami
BC
Pitt
Syracuse
Cal
Stanford
Traditional XII schools you'd keep:
Arizona
Arizona St
BYU
Colorado
Iowa State
Kansas
Kansas State
OkSU
Texas Tech
Utah
WVU
That's 20 right there. Could you maybe look at subbing out someone for SMU, TCU, Louisville, Cincinnati, Wake, UCF, etc? Maybe. Could you sub out someone like Boise St for one of the above? Maybe.
But I think you could get to a solid 20. It might still be 3rd of 3 in the actual pecking order, but over time as you're not diluting with schools numbering 61-13x for recruits, things might rebalance a little as some of these schools could start pulling in recruits. And you're in geographies that the B1G and SEC wouldn't be in, so there might be some local recruiting advantages for some of those schools. They might not be the most fertile fields, but you'd be unchallenged in AZ, KS, CO, UT, upstate NY (and frankly much of the Northeast might prefer Syracuse to anywhere else) and the Bay Area.