number of snails

I would start with one or two and see how they do over the course of a month or so. Then add one per similar period of time until there is a reasonable level of algae control and the population stays stable--i.e. you don't lose any to starvation. Even then, unless your tank is heavy with film algae, I'd say 2-3 would be the maximum. Even that would be pushing it for the average tank.

i would do 2 mexican turbo snails since they're the only snails that seem to do anything. thats only if you have an algae problem though.