new tank? old tank?
new tank... stay away from snails unless you have a lot of coraline growth on the rocks.
old tank... then you wouldn't be needing to ask this question.
start with something very small. 3-5 hermit crabs (red legged/blue legged/zebra/scarlet/halloween/white tipped whatever you want)
some hermits, variety of snails - do you have green hair algae issues on glass/rocks??
start with basics and up the quantity when you feel you need more...