I'm going to stand out in oposition to the others, I don't think that you should use the rule of thumb, for snails anyways, for hermits 1 for every 1-4 gallons is fine, but for snails you can't really go by that, every different tank has different perameters, I have a snail and a half for every gallon and by the evening there are swirls of algea left on my glass,but my friend Colin has one snail per four gallons but his glass is clean as can be and I'm sure his snails are staving.
Also pay attention to the type of snail, margarita snails are awesome snails, they work hard, they rarely knock frags over, and they can right themselves if they fall, however if the tank temo is over 72 then they die sooner than they have to, astreas like cooler water aswell and are unable to right themselves half the time, a good rule of thumb is that a snail with a boney trap door won't be able to right itself, but if it has a horny trap door then it will be able to. I personally love my nerite snails, they are hard working, never knock frags over and stay small, I'm actually going to order 5 more for my 24 gallon in two or three weeks.