From ESPN:

In order to account for student-athletes who will have to miss games because of positive coronavirus tests, contact tracing and regular injuries, the Big 12 determined on Friday that teams with at least 53 players available--including seven offensive linemen, four interior defensive linemen and one quarterback--must play or it will be considered a forfeit.

If teams have fewer than 53 players available and don't meet the position benchmarks, the game would be declared a no contest if it can't be moved to a later date. Teams that fall below 53 players--but are still able to meet the position benchmarks--have the option to play the game.

"It's not intended to eliminate teams as a result of how many players they have," Big 12 commissioner Bob Bowlsby said. "It's intended to determine the outcome of games based upon your ability to prepare for the game and execute the game plan."

Sources in the SEC and ACC told ESPN both conferences are working to determine similar benchmarks for a minimum roster size.

Considering the NFL has 53-man active rosters, this seems to be a reasonable threshold. In effect, it allows a team to go 2-deep at 25 positions (including PK, punter and long snapper).