You can put up to six neon tetras in a 10-gallon tank. One neon tetra requires about two gallons of tank water to have enough swimming space.
Neon tetras are schooling fish that get stressed and die if there are too few of them. The ideal amount of neon tetras you should keep is 10, and 10 neon tetras require a 20-gallon tank.
Also, these fish friends love floating plants, so make sure you provide enough for them (the more, the better).