Generally it's not a good idea to have a mixture of hermit crabs and snails. What will happen over time is the snails will be *slaughtered* by the hermits for their shells. Sometimes for no reason at all. You will find just empty shells scattered about. There are some less aggressive hermits though. You could house scarlet reef hermits, or right hand hermits with snails.
I personally like snails for algae cleanup/sand sifting, and hermit crabs for leftover food/waste. I would decide on what you will be aiming for with the cleanup crew, and what you can take care of on your own. If you can manually scrape algae off the glass and such, maybe go for the hermits. If you won't be overfeeding and not needing to worry about food cleanup, I'd stick with snails.
Now, onto the point of how many for a tank that size. I'd say 1 hermit/snail for each 2 gallons. That way, you won't have them dying off if they aren't getting enough food. Maybe start off even slower, and overtime add more if you think the crew isn't working up to standards.
I personally favor snails over crabs..(shh! don't tell my scarlet reefs)