depends on how often you feed them and how many water changes you do. Also what type of filtration is on the tank.
if you don't want to breed them and you feed the fish once a day and do a 50% water change each day, you could have 50 adult platies in that size tank. However, if you put a little bit too uch food in the tank or missed a water change, you would probably lose most of them.
I would say about 10 adults in a 10g tank and that would allow them to produce a few young and you could feed them once or twice a day and do a partial water change each week.
Monitor your nitrate levels and if they are going up rapidly then do more water changes, or reduce the food or number of fish in the tank.
The same applies to PH, if it goes down rapidly then do more water changes or reduce the load on the tank.