Start with 3, wait another week and see if you can see any progress, and add more if you see fit. For as long as there are food to eat (including aiptasia), peppermint shrimps are pretty sociable and can be kept in much higher density then fish, not too extreme of course. Even if all aiptasia are eaten, they can be great scavenger.
Make sure your tank does not have yellow polyps, otherwise, they will be eaten too.