Hold on. There are several types of turbo snails out thre. Most are good at eatign alage. Some however eat different types of alage and they come in a varity of sizes.
For a 50 gallon I think 5 turbo snails would work fine. I like the mexican turbo of a larger size cause I have seen these guys clear rock that was infested with green algae within a few days.
Trust me get less than you need and see what happens. If you get too many what happens is they clear up the alage and then start to starve to death.