If you have females only, a pair would be fine. If you have a male, it is best to have a trio of a male and two females- in a male/female pair, the female might be chased a bit by a lustful male.
If you want two make sure you get either two males or two females if you don't want to breed but they do better in pairs. I have three platies 1 male and 2 females no fry yet.
Just let them breed===The frey are good food for the rest of the tank (sorry, sounds cruel, but true). If you do not try and save the freys only a very few will survive and not over populate the tank.