It all depends on the size of the tank. If its 2.5 gallons, you can have 2 guppies, but make sure that the guppies are the same sex so they don't breed. If you have a 5 gallon tank, you can have 3- 4 guppies, but you shouldnt get them to breed. If you have a 10- 20 gallon tank you can breed guppies, but you have to get 1 male and 2 females becuase if you only have one female, she will get stressed and die. You will have to get a breeding tank. They are only like $4 at pet stores. When you sont want to have anymore guppies, don't use the breeding tank and the other guppies will eat the babies.