You can put up to five platies in a 10-gallon tank. Platies are tiny, growing to about 2.5 inches in length when mature. Because of their small size, they don’t need a lot of space to thrive.
As a general rule, an inch of fish requires a gallon of water in the tank, so a platy only needs two to three gallons of space. With that in mind, five platies can live comfortably in a small, 10-gallon tank.
However, keep in mind that this rule only applies when you’re keeping a species tank. If you want to build a community tank, you also have to account for the other tank inhabitants when deciding how many platies to keep.