Their multiplication depends on nutrients. Many nutrients=many snails(generally speaking). A slow reproducer is the assassin snail due to the fact that it is not a hermaphrodite and cannot switch sexes to breed, it requires a male and a female to reproduce. Even with that, you will not be overrun with them any time soon as they are single egg layers. Quick multipliers include the common pond snails, ramshorn, malaysian trumpet, but again, it depends on nutrients. If you have a lot of dead plant matter and you feed your fish a lot and don't gravel vac, etc, you will have loads of them in no time.
