You can keep two to four mystery snails in a 10-gallon tank. A fully-developed mystery snail requires 2 to 5 gallons of space to live comfortably. These gastropods are small and slow-moving, but also highly active, so they need a lot of room to explore. Therefore, a 10-gallon tank can only house up to four mystery snails.
However, note that the previous rule only applies to a species tank. If you intend to keep your mystery snails in a community tank, you might need a bigger tank, depending on how many other species you keep there.