Obviously this depends on the programs that are available. But if for some reason Notre Dame decides to join the B1G and convinces them to bring Stanford along to get to 20. And if North Carolina joins the SEC and brings Duke along (to retain the best rivalry in college basketball) with Virginia & Florida State, that leaves quite a few high quality options on the table for the XII.

Miami, Clemson, Virginia Tech, NC State, Louisville & Pitt would all be significantly additive football programs to XII revenue and highly coveted. And you'd still have Georgia Tech & Syracuse on the East Coast and a few options out West (SDSU?) to get to 24 depending on regional preferences for conference construction.

Now if the B1G & SEC stop at 20 by grabbing UNC, UVA, Clemson, FSU, Miami, and one of: VT/NCSt/Duke, then the XII should limit expansion to 20 with NC State & Virginia Tech (if available), along with Pitt & Louisville. Georgia Tech could replace either VT or NC State if both aren't on the table.

The reality is that the XII needs all the football legitimacy it can get right now. And if there are quality football programs on the table, the conference has to grab them while it can because CFB realignment history tells us that conferences viewed as "lower" will inevitably get poached. And you need a deep roster of quality football programs to survive as a perceived power conference long term.