I reaffirm what jh1987 said. Also, it helps to know the fish's age because the number of fish also depends on the level of maturity of the fish. If the fish is really young, it hardly gives any and the same goes for really old ones (average longevity is close to three years) but the mature fish give quite a few. My platy gave birth to 18 or so, but then died, so I'm not sure if she was young or old or her gestation period was sort of off...