For both of you w/30g tanks, I'd do 3 nassarius (they'll live under the sand and come out at feeding time - awesome lil buggers IMO
), and maybe 3 astreas ( 'cause they're likely easy to find locally), and 3 ceriths. Ceriths don't turn over as easily as astreas. They turn over and they'll die. Nassarius snails can do backflips.
Some don't like to mix hermits with their snails as hermit can kill'm off for their shells. 2 things about that; 1) put some empty shells in a bit larger than your hermits, and 2) thats what crabs do. I'd just update the snails from time to time.
Anyway, that's the number I'd put in there. Others opinions may vary, but you don't want too many in new tanks since they'll likely not have sufficient food sources yet