Apple/mystery snails need 2 to breed (a male and female). So if you get one, they won't breed.
Get two, they still may not breed, I believe they like high humidity to breed.
Check out this website, its really helpful:
You may want to start off with just one, because some bettas won't tolerate having it in their tank. Better to get one, then get like three and have your betta kill them all.
Edit: In some ways, they are worth it. They are quite funny to watch, and they move quicker than I thought they would. My snail, Acrobat, likes to do acrobatics, he climbs up the heater and somehow ends up on the plant beside it, and then climbs up the filter and back down, and around..... They DON'T clean, if anything they poop more than they help clean. They are more for entertaining purposes than cleaning.