It's free food to the tank. That's why I like to have shrimp in the tank too, and yeah, ever try to feed a coral with shrimp in the tank? Good luck with that!
They are simultaneous hermaphrodites, so both male and female, but, they cannot self-fertilize. So you need 2, but any 2 will do. It's actually better to have them in small groups but you usually need to introduce them at the same time. They are a socially structured animal believe it or not and the social dynamics/interactions are very interesting to observe.
They mate when they molt.. if you get to see one molt you'll usually see the shrimp go into a mating frenzy. A day or two later you'll notice one of the shrimp will be carrying eggs.
Once they start, they more or less continually carry eggs all the time.
The babies hatch at night (shortly after lights out) and look exactly like miniature versions of the parents. But they rarely last more than the night unfortunately. Apparently they taste pretty good.