erm hmm there is a rule but i keep forgetting it, but it really depends on the size of the fish you get, i.e small fish you can have loads, big fish not many...
you could possibly get some mollies, maybe guppies depending you have loads of hiding places, maybe a male betta, a pair of rams? a pair of gourami's... school of zebra danios, leopard danios, assorted danios, neon tetra's, cardinal tetra's penguins, rummy nose tetra's glowlight tetra's black heart tetra's...
basically if you want an impressive shoaling fish, then look at some tetra's of all different colours...
are you new to fish keeping?