One snail in a fish tank can multiply by itself if the snail is a hermaphroditic species like Malaysian Trumpet snails, Ramshorn snails or Bladder snails.
These hermaphroditic species of snails can multiply by themselves because they possess both female and male reproductive organs, which allows one snail to lay fertile eggs and also create a massive population without needing a mate to reproduce.
Although not all snails can multiply by themselves.
Common "hitchhiker" snails if you have a single bladder or ramshorn snail for example can turn into dozens or hundreds of snails.
These hermaphrodite snails can fertilize their own eggs, which are also often laid in small and jelly like clutches on plants, decorations or even tank glass.
There are some exceptions though as mystery snails like those in the family Ampullaridae have distinct genders and also require both a female and male snail to reproduce.
And to prevent a single snail from becoming an infestation, you can reduce feeding to eliminate excess sources of food.
Also some snails like the Nerite snail can also lay eggs, but they will not hatch in freshwater.
When you have snails that are multiplying in your fish tank, the snails multiplying and snail outbreaks are usually a response to available food such as uneaten fish food, dead plant matter and other organic debris.
If you have a snail outbreak in your fish tank, reduce feeding, do more frequent water changes, vacuum the substrate and clean your filter more often.
To stop snails from breeding, you must control their environment by reducing food sources, limiting hiding spots, and manually removing eggs.
Because most snails are hermaphrodites that can self-fertilize, you cannot prevent breeding by only having one snail; you must actively manage their population by identifying and destroying eggs.