Well considering I have 5 in a 1 gallon happy as can be, I would say at most 13. 14 if you really want.
They are very small, and have an almost nonexistant bioload, so bascially you could have 100 in a 1 gallon and you could still have 0 ammonia and everything if you did water changes.
Keep in mind- They are HIGHLY sensitive to ammonia. Even the slightest trace can kill them.
*2 Bettas (for now RIP Ryu) *2 Goldfish *9 Guppies *1 Gourami *2 Chinchillas *2 Budgies *2 Dwarf Frogs *1 Dog *1 Cat *2 Hermit Crabs *2 3gs *1 10g *1 Pond *2 2gs