This depends on what spec you are talking about. For healers and tanks, the number of people and composition of healers in the group makes a difference. For DPS, raid composition doesn’t affect anything.
We run raid healing sims with 20 people and 4 healers. The 3 “other” healers are a resto druid, mistweaver, and holy priest. For tank sims the healers are scaled back pretty far to allow the tank to do as much of the work of keeping themselves alive as possible.