It sint really a question of how many it depends on what your thinking of, Obviously there are many fish that will grow too large for a 30 gal tank. But you could have clowns, a royal gramma, small gobies, damsels, chromis.. not all but you have to get an idea of at least a few youd like to keep for others to help you more
I think how many fish you can keep depends on the type of fish and your individual setup. My 30g is 9 months old and I built up the population gradually, watching the levels and leaving time for things to adjust between additions. There's a little HOB refugium on the tank filled with Chaeto macroalgae and 30lbs of LR which together keep NO3 and PO4 at undetectable levels.
I keep six fish: a Lubbock's fairy wrasse, a yellow clown goby, an ocellaris clownfish, an orange-spotted shrimp goby, a white-tailed dwarf angel and a Talbot's damsel.