21 starts is the number I use for everyone and I've never had one complain at 21 unless it was because I benched them too many times early on (so the ratio fell way below 80% at that point). 22/24 is definitely safe to bench for the last two games imo.
You might can even get away with using 20. Sometimes I'm in the position where they've started 20/25 going into the last game, and I'm not sure if they can complain after game 26 or not. But I always start them in that scenario just in case, since I can't make up for it after that if they do get unhappy.