Right now I have 6 week old platy fry in a 5 gallon tank... About 25-30. They are just now beginning to show some black coloring on the tips of their tails.
As for the fry in your community tank, I don't know how many you could have. Remembering my platy fry when they were born, I could barely see them unless they were darting across the tank.
It takes about 4 months before they're going to show color ( according to my LFS
guy). My other Momma platy gave birth in my community tank and I have no survivors. I couldn't handle 50+ fry so I let nature take its course this time.