I prefer a mix of species like Mexican Turbos, Trochus, Margaritas, Ceriths, etc. so you have your bases covered on different types of algaes and surfaces (ie. rock, glass, etc.). Species that will reproduce readily in your tank like Stomatellas are also nice. They provide some natural food for the tank as well as self-sustaining populations. If you have a sandbed some Nassarius would be good too. Conchs are good too but I don't prefer them as they get large quickly (avoid Queen Conchs IMO - they just get too big too soon) and have a tendency to knock frags over because of their size and the way they move.
IMO how many really depends on the tank - every setup has different needs depending on how it's run, types of algae present, nutrient levels, etc. Also depends on which species - obviously larger species eat more so you don't need as many. I prefer to start out with a relatively small number of each and see how it goes. I'd rather add a few more later than have too many and starve a bunch of snails to death over time.
JMO...