Yes. You have the amount you need.
Their population will vary with how much food is available. If you don't want as many, you might try feeding a little less, or get an arrow crab. I think they about the best clean-up crew you can have. If something dies, or you feed a little too much, they take care of it before it creates a big problem. They might be ugly and a bit of a pain if you have your hands in the tank, but they are worth it in my opinion.