About 40 or 45 of various small fish under 1" that school, like some Tetras, Barbs and Minnows or up to one medium large fish like a Pleco or some type of Cichlid.
There's a lot more then that but you should get the picture.
The numbers and combinations you can have are almost endless ranging from one aggressive medium sized fish to several schools of small fish, also the decor you wish to have will dictate how many fish you can have, in a tank which has a lot of wood or rock work you will be able to have less fish than a tank with just one or two pieces as more of the water volume will be displaced.
Whatever you choose to do with the tank remember to add fish slowly so as not to overload the biological filter and keep within the safe stocking limits, the general rule of thumb that is advised for most new fish keepers is to not exceed 1" per gallon of water but this rule only works with small community type fish.