yea...sometimes they get eaten as soon as they are born. The only way to avoid it would be to place the mother in a birthing nursery before hand, or place some moss or plant cover so that the fry have more hiding places.
If you want to raise the fry platies have fry every~30days approx. You will notice that their bellies get very full and square off a bit towards the tail. Sometimes you can even see a goldish shimmer at the base of the belly days before they are about to drop depending on the platy's colouring. Mostly you can tell just by the shape of their bellies. They will drop around 8-15 fry depending on age and some other factors.
I recommend getting a birthing nursery with a v shaped floor that has a separate chamber for the fry to drop into. This way the mother cant get them. Platys usually drop very early in the AM while it is still dark, so watch for the fry in the morning and be sure to feed the mother to keep her healthy. Giving birth is hard work.