That depends entirely on what fish you have in it. A couple of general points:
you should be looking at fish of not more than 2-3 inches adult length (and make sure you research adult length, as most fish in the shops are babies)
you should avoid VERY active swimmers (e.g. swordtails) if the tank is under 3 foot long
if you choose a schooling species you should keep at least 6 in the group
if you have several species, make sure they are compatible in terms of temperature, water type (hard-soft, acid-alkaline)
check if any of the species you are interested in is territorial: this may limit you to one of a particular species
if you stick to 2 inch fish, a very rough estimate is to allow for 15 inch of fish in a 60 ltr (15 US gallon) tank
so this would allow you e.g. four 2-inch corydoras and a school of 7 smallish tetras
