You could have guppies, Endlers, platies. The other livebearers (mollies and swordtails) get much too large for this tank. As for numbers, it depends which, but three male platies and three or four male guppies should be OK if you are regular with your weekly water change of 50-60% of the tank volume. Having live plants helps this too, and floating plants are ideal with these fish as they like to remain near the surface.
Male and female is easy to ID with these livebearers. It is advisable to stay with only males; once impregnated, a female platy or guppy can produce regular batches of fry and not all will get eaten, and before long you have a way overcrowded tank and more problems. Endlers are not so bad for this as they are less productive. Endlers and guppies will cross however.