It depends on the Snail. Mystery Snails grow big so they need at least 10 Gallons. The Betta could live with them in a 10 Gallon, though. A Nerite Snail can go with a Betta in a 2.5 Gallon, but that's not ideal in my opinion.
Over hundreds of bettas die in their small, dirty cups each day. Only you can help them live. Research and then save them, today!