The "general" rule of thumb is 1 "cubic" inch of fish to 1 gallon. (From what I remember.) People can get super crazy with calculations in the fishkeepers world. Some fish, by their size, produce less waste, or more waste and require more or less filtration. Others are less active fish and can be kept in smaller spaces, or vice versa. Aqadvisor isn't perfect, but definitely helps with general guesswork.
Edit: Make sure your filtration is up to snuff. I haven't kept platys, but if they're anything like mollies they're pigs.