Pond snails are a type of bladder snail and reproduce asexually. It only takes one. Keep the tank clean, do not overfeed, and they will be self limiting. If they over produce you can find someone who needs feeder snails on here and give way or sale the excess. Lots of people buy assassin snails to get rid of the other snails and then have to buy other snails to feed the assassins when they do their job. LOL