How many goldfishes can you put in a two and half gallon tank?

You should not put any goldfish in a two and a half gallon tank. Goldfish need a minimum of 20 gallons of water per fish, and they produce a lot of waste, so the water needs to be filtered and changed regularly. A two and a half gallon tank is too small for even one goldfish, and the fish will likely become stressed and unhealthy.