I ordered malaysian trumpet snails off eBay; used top seller from USA, to assure freshness and health. I have a sand bed and I ordered min 24 for my 6gal long tank. Sounds like a lot but I hardly see them. Only have a few babies; they tend to self regulate as long as water params are good and there's not too much excess food/mulm. They clean algae of all sorts. They eat leftover food and fish waste, and my IAL after they start to decompose. They also keep my sand bed aerated. Depending on type of snail and purpose would determine how many. Think general rule is 1-5 per gal though.
The more I think I know, the more I think I don't know much at all!