There should be at least 2 at each school that you have an interest in.
I agree with this, wholeheartedly. Just because a school is a safety as far as you're concerned, it should still be a place where you would like to pursue research. If it isn't, in my opinion, it shouldn't be on your list. As such, you need to ensure that there are people doing research that you're interested in. As bfollinprm mentions, there should be at least 2 such profs because there's no guarantee once you're admitted that you're going to be able to join your first choice group.