Platy Giving Birth

Giving birth does take it out of females. Just keep an eye on her to see if she is pestered by any males.
There were probably quite a lot of fry born. There may be others in hiding especially if you have a lot of places/plants for them to hide in. A lot would have been eaten though. It sounds callous but if you try to save every fry that is born there won't be room left in the tank. I would just leave the fry you've found in the tank. Maybe crush up some food very small so it can find tiny particles to eat rather than big chunks.