Hi and welcome! Just double-checking: how was the tank cycled? If you just let it run without adding an ammonia source it isn't cycled I'm afraid.
A 20 gal is a decent size. There are some options, too many to name off-hand really. I'd suggest going to local fish stores and write down the names of fish you like and then research (or/and ask here). Just don't let them sell you anything lol
For an example, you could do something like:
A group of 8-10 schooling fish like neon tetra, rummynose tetra, harlequin rasbora, etc (many schooling fish are options here but not all, depending on size)
A group of 6 corys for the bottom, like Panda, dwarf or pymgy
A centerpiece fish like a dwarf or sparkling gourami (don't get the large type of gourami)
Shrimp
As for how many fish, there isn't an exact way to calculate this. The old rule of 1 inch per gallon is antiquated as it doesn't take into account things like fish's activity level.
Here's a guide if you haven't seen it:
Guide to Starting a Freshwater Aquarium - Aquarium Advice