This question has a complicated answer. There are several things you have to keep in mind and consider.
First: What species and size/age are you thinking about?
If you're thinking about larger adults, it's not going to turn out well. Mixing species probably won't go well either. Especially if you're mixing some of the crankier ones. IME, A. ocellaris seem to be the nicest ones, but you never know.
I have seen people start out with a group of juveniles that all come from the same batch and that "can" work, with caution.
Second: Just because you have anemonies doesn't mean they will host in them. Be sure you have the proper lighting for keeping 'nems too.
I wouldn't suggest doing more than one pair in a tank but if you do I would go with juveniles and add them at the same time. If you don't, one pair is going to have the home turf advantage and probably be quite aggressive.
As an anecdote, I can keep 50-100 juveniles in a 20G tank for a while
but one fish will always start to take over the group and have to be moved to another tank. By "juveniles" I mean under 1/2".
Give me a shout if that doesn't answer your question.