Dwarf puffers are freshwater. 1 is good in a 5 gallon. Definitely no more than 1. I've tried 2 in a 5 gallon and the maintenance was quite a bit of work. They're also territorial so it's better to give them more space. Hope this helps
According to what I've read on the dwarf puffer forum, unlike other puffers, they don't need snails to trim their teeth so they're fine without snails in their diet. Mine live mostly on frozen blood worms. They're very interesting fish
Edit: Oh and from the way I've seen them eat snails, they more of suck the snails out of the shell rather than bite.