Common pond snails as already mentioned are egg-layers. In tanks with certain fish (especially barbs) you might not see too many of these since the eggs are tasty little morsals to many fish. They do not burrow in the substrate like MTS do. They are some (if not THE) fastest snails around and can quickly zoom from one side of the tank to the other.
They are quite hardy and will populate tanks fairly quickly when a food source is present. Billions!
would generally denote an overfeeding or poorly maintained tank. They will populate to the amount of food present. I would up the cleaning or lower/change the feeding amount/type. While nice to have in the tank and very interesting to watch for some (myself included), a ton of them are a general indicator that you might have some nitrAte levels to watch out for.