You can have up to eight guppies in a 10-gallon tank. Generally, a one-inch guppy requires one gallon of water in the tank. Male guppies typically grow to be 1.5 inches long, while female guppies can grow to be 2 inches long. Therefore, you can have either eight male guppies or five female guppies in a 10-gallon tank.
If you plan on keeping male and female guppies together, you should keep a ratio of one male for every two females. If you follow this ratio, you can have two males and four females in a 10-gallon tank.