A P12 merger would be dumb. Why take on their dead weight? If the ACC would even think of adding P12 teams it should only be Washington, Oregon, Cal, Stanford, Arizona, Utah, Colorado. The other 3 schools are useless.
That would give the ACC 21 teams. If you take 3 more from the B12, you can get to 6 pods of 4 schools. That will allow regional scheduling in non FB sports.
West- UW, Oregon, Cal, Stanford
Rockies- Arizona, Utah, Colorado, Kansas
South- FSU, Clemson, GA Tech, Wake
Carolina- UNC, NC State, Duke, UVA
North- BC, SU, Pitt, Miami
Appalachian- Louisville, VA Tech, Cincy, WV
You probably could add a Texas pod too of Houston, Texas Tech, Okie State, and Memphis. That would kill off the P12 and B12. ACC would be a clear #3 and if it lost members to either the B1G or SEC they have plenty of schools to fill in any holes.
That being said the largest big conference add thus far has been 2 teams. A big add has yet to happen.