how many guppys can be in a 20 gallon tank?

Are there any other fish in the tank .

Ideally 5 males as if you mix male and Female you will get fry so i would look at 5 and then some other species of Fish Maybe Tetra ?

Is there any other fishes in that tank? If not, you can keep 12 males maximum.

with no other fish in the thank, about 12 can be held.

Put 9 males and 1 female. So each can bust their load inside, and make her explode.

Depends how alive you want them to be.