I'm not sure how they breed, but once in a long while, I'll see a baby one crawling on the glass. I've never seen any eggs, though it could just be that they laid it somewhere I couldn't see. Given the rarity of the baby snails, I'm also assuming that they probably only give birth a few times a year, and to only a few young.
If you find more information on them, do share. I'm down to my last few and would like to get a better idea on breeding them.