Mollies do thrive in brackish water, but they live perfectly well in freshwater tanks, and they have similar needs as guppies
Either way, you could get a bunch of male guppies if you don't want them to reproduce, because they do, in a crazy speed! xD Other than that, you can easily calculate the stocking level by giving about 2 liters of water to each 1cm of adult fish body. Which is about 1 gallon per inch
It's different with guppies and other small/shoaling fish, where you could even think 8 inches per gallon. Guppies grow to be about 2 inches in length, and mollies to be around 3-4 inches.
In a twenty gallon, you can typically fit 20 inches of mollies or
160 inches of guppies. So you could fit 5 mollies and around 20 guppies if you like
It could be good to have some bottom dwellers in such a tank as well! Like a few corydoras or common ancistrus would stay pretty small
Then it would be good to have maybe... 2-4 corys and 1-2 ancistrus. The ancistrus will be 4 inches, and the corys will be around 2-3 inches when grown. So if you would want the bottom feeders, you could fit 2 corys and 1 ancistrus + 3 mollies + 12 guppies. How does that sound?
But, you shouldn't get all the fish at once, remember that! You could start with getting all the guppies for a start so the filter will get used to fish
After a week or two with good water quality, you could buy some mollies