They always say that a snail needs at least 2.5 gallons each,and that depends really on the type of snail you get.Brigs/Mystery snails tend to get about the size of a quarter,where Canna's,Haustrums,etc,get between baseball /softball size,so you have to consider that when picking fish and the size of your tank.
A big enough tank and alot of filtration is a must along with perfect water params.
As Selena mentioned,these snails are not really the best for keeping your tank cleaner.They have a lot more requirements then most people realize,including a lot of fresh vegetables needed on a daily basis,calcium supplements,etc.They really only eat algae as a last resort,when they can find no other food alternative. You would be shocked when owning a few of them to see just how much waste they can produce.

When I kept snail tanks,all I did was syphon poo.