How many fish can you have in a 20 gallon tank?

not many. Until recently i had a 15litre 'Baby Biorb' bowl with 2 goldfish in, however it did seem quite cramped even with just those 2 in there so I have upgraded to the 30litre version.

i guess however that it depends on what type of fish you are planning on keeping, i would say that my 15litre bowl would house maybe half a dozen minnows without problems, but anything larger will just feel cramped and won't be good the for the fish's health.

hope this helps to answer your question The rule of thumb for freshwater tanks is 1 inch of fish per gallon. That'd convert to 0.7 cm per liter if you have metric fish.