Good morning!
Trapdoor snails aren't "good" or "bad" for your pond - they are just another creature that may enjoy living there. They do eat algae and debris off the bottom, but honestly, not to any degree that I've ever said "wow! I should get more snails!" How many? Totally up to you. I don't know that there's any formula to that question. I bought 20 the first year we had our pond and probably still have about that many. We've found a few dead outside of the pond over the years - raccoon snack probably - and a few died right after we put them in, so we've maintained pretty much the same population. They are live breeders, but they multiply slowly.
I would be careful about your source when you buy them - make sure you're buying them from someone who knows how to handle them so you don't get a bunch of dead snails showing up in your mail. If I were you I'd find a local source. I like to be able to see what I'm buying and see how they've been living before they go in my pond. Stores that sell aquarium supplies and fish often have them - at least here in the US - and they may be willing to get them for you if they don't.