Most people say minimum 1 per 5 gallons, but I'd probably be keener to keep 2 in a 20 gallon, mainly because of the massive horror stories you see about how too many puffers in a small space eat everything in sight.
Sir minion is the puffer hero on these forums from what I've gathered in my short tme here
If the tank is well-planted, I think 3 would work in a 20-gallon.
This might be a stupid question, but when you say "with a couple of snails" -- are you talking about snails for food, or snails as in apple-snails or something that you want to keep as pets? Snails are a favorite snack of puffers, and I'm sure they'll thank you for the tastey meal!
Actually, before I got my two DPs I read somewhere that they would only eat small snails, but I've been surprised to see them clean out snail shells larger than they are. Quite a meal!