It's one of those "how long is a piece of string" questions, really. But a couple of suggestions can be made:
no fish bigger than 4 inches max, 1-3 inches is better in most cases
no very fast swimmers
tank is not big enough for separate territories for most fish, so only one strongly territorial fish (or in a few cases you may get away with a pair)
IF you go for two-inch fish that are not strongly territorial and not exceptionally active or boisterous, you might fit in 8-9 two-inch fish, of platy/cory size, say. Tiny schooling fish, you could fit in more. One suggestion might be:
school of 6 harlequin rasboras, or black neons, and 3-4 smallish corys.