It could be debated wether or not you "need" snails at all. Im sure they would'nt hurt. I suppose it would depend on the kind of snail you want...you could start with one or two and in a months time end up with 50. Some snails will multiply faster than you can keep up.
I personally have quite a few snails in my 15 gallon community tank. I inherited them when I bought a plant from Petsmart
They mulitply a bit and I just keep removing them as they become too much for my liking. I think some of my fish are actually consuming them....I had all the fish removed from the tank for a week and the snail population quadrupled from what Im used to.
I also have a 1"-1.25" Gold Mystery Snail. Now this dude is just plain cool. He hasnt multiplied; I believe that the Mystery snails are NOT asexual so you have to actually have a male and a female to copulate, which is a good thing imo
. He likes to nibble on my plants a tiny bit, but has really made short work of my Radican Sword. Im not sure if the thing will grow as fast as he eats it.
I cant say if he really does much good as far as eating algae and junk in the tank...Ive seen him eat my plants and shrimp pellets, so Im not sure if he ever has eaten algae. I also have a dozen ghost shrimp and 2 cory cats in the tank that really do a number on cleaning up the leftover junk on the bottom of the tank. So far, the snail is just cool to look at. In all, I would certainly think that 2 of the big mystery snails in a ten gallon tank should be fine. One is neat to look at, but I guess I wont really know how well hes keeping my tank in order untill hes gone.