At least six neon tetras should be kept together in one tank. Neon tetras are a schooling species, so you should keep a minimum of six to ten neon tetras together in one tank. Neon tetras will feel uncomfortable, get stressed, and maybe even die if you keep too few of them together.
Additionally, neon tetras are active fish that require plenty of swimming space. If you have more than 15 neon tetras, make sure you have at least 20 gallons (76 litres) of water in your tank. Also, be sure not to overcrowd your fish tank.