I have red-leg & blue leg hermits in my tank. Probably 25 or so right now and always have had them. They do hijack a snail here & there, but it's not that bad. Emeralds are little buggers though.
Your CUC depends on your inhabitants, age & feeding. You could do fine with anywhere from 20 snails to 50+. Start with 20 and a fighting conch and see where it goes.