This is one of those questions that is framed wrong, like "How many times a day should I feed?" (There is no correct answer, because it's the
quantities involved that determine the frequency.)
Ditto the number of fish. It's not the number of fish, it's the SIZE. (Insert "size matters" joke here.) You CAN house five fish, perhaps as many as seven, IF the species are right. They need to pico-size fish that will not exceed an inch or so.
Look at Trimma cana, Trimma rubromaculatus, Elacatinus oceanops, Elacatinus multifasciatum, Elacatinus puncticulatus, Eviota pellucida, Eviota nigriventris, Discordipinna griessingeri, and Coryphopterus personatus, just to name a few.
SPS -dominant 20 gallon tanks are possible, but extraordinarily challenging for a beginner, especially without a sump. Be kind to yourself and start with less demanding corals.
I would stay away from anglers and dwarf lions in that tank.