I have got to sit down and write a long post on this one of these days....
In a heavily planted tank: (Not All At Once!!)
Maybe 8 of the somewhat larger tetras - Black widows, Diamonds, like that
Up to a Dozen fish slightly larger than Neons: Glowlights, Harlequins, Black Neons, Lemon Tetras.
Up to 15 fish of the same general size, shape, and demeanor as Neon tetras.
Up to 20 Ember Tetras, Green neons, or Microrasboras (Fish considerably smaller than neons)
Up to 6 kuhli Loaches
Up to 15 Pygmy, Dwarf, or Dainty cories (C. pygmaeus, hasbrosus, and hastatus, respectively).
Up to six small cories (C. panda, san juan, guapore - the ones that max out around 2" - no larger than that.)
One male betta
6 female bettas (no more, no less)
Up to 15 Amano Shrimp
Up to 20 Red Cherry or Crystal Red Shrimp
Two or three Sparkling, Dwarf, or Dwarf Honey Gouramis
Up to 3 Ottos (you won't be able to feed more than that)
A half dozen guppies (and a will to cull the fry)
5 Platies (or more dwarf platies)
Up to 10 Endlers
2 or three dwarf puffers (and nothing else)
A half dozen badis badis (but you'll need to keep live food on hand)
A single pair of the smaller apistos.
2 or three moth catfish (good luck finding them either to put in or to take out)
Up to 4 Mystery snails - but best to keep it to one.
Probably as many as three African Dwarf Frogs.
In a Tanganyika type tank, one breeding group of shell dwellers (either a pair, or a male with a few females - and lots of snail shells)
In a brackish tank, up to 10 bumblebee guppies
In a cool water tank:
up to 12 white clouds or
one male and 2 female white cheeked gobies (no more, no less)
So a really nice little tank could be 8 or so ember tetras, a pair of ottos, 8 or so Pygmy cories, and say, 4 or 5 Lemon Tetras
Or 5 kuhlis and a pair of Dwarf Honey Gouramis.
Or one male betta, a mystery snail, and a couple of dwarf frogs.