20 Gallon Tank Stock

[boberga]

Currently have 3 guppies 2 mollys and 1 platy in the tank. How many more fish can I add to the tank. And what combinations?
Tank is has natural colour gravel 4 fake plants tetra whisper pf20 filter and a tetra whisper 20 air pump heater set at 79F and a LED hood.

 

[Byron]

You are pretty much at the limit now.  The mollies have a size expectancy of three inches for males, and five inches for females, with some sources mentioning six inches possible.   There is also the issue of fry, if females are present and have been impregnated already.  Same holds for the platy.
 
I know it seems like these few fish have plenty of space, but at their mature size things will be very different.  Aside from the issue of physical space for them to "be normal," there is the issue of water quality as they develop.
 
Byron.