The number of fish you have is fine, for now. You need to change half the water in the tank every week and get some floating plants too. Anacharis and Pennywort are best for Livebearing fish. Also, you need to add a teaspoon of standard aquarium salt to every 5 gallons of your replacement water. Livebearing fish will be healthier with a little salt. No table salt, it has additives that might harm your fish.
Livebearers are fast breeders in pure water conditions, so you need to make plans to get a larger tank, at least a 30 gallon and a 55 is best.
Just one reporter's opinion, though. Waterkeeping isn't an exact science and there are many ways to succeed.
"Fear not, my young apprentice. Just change the tank water."