Again it depends on how you cycled it? If you used pure ammonia and dosed to 4, then you can add them all at once. If you cycled with fish then you want to add fish slowly (one at a time) to allow your bacteria to adjust. At most I think 4 platys for the tank that way you'll have a bit of room to add a bottom-feeder later on. Also if you get a mix of males and females they'll probably breed and your tank can become overstocked very fast.