You'll probably need that many once you have all of your other livestock in there. I'm kinda new to the hobby, so I might be wrong. I'm adding my clean up crew as I go. I started with about the same number that you have (with no fish) and am planning to add more as I add more fish.
The way I figure it, is that there isn't enough to eat in there for a full clean up crew and I'll starve some of them.
There are lots of different snails that eat various algae and left over food. You will need to see if you really do need more snails, in my 45 gallon corner tank I have 2 snails and no crabs, that is all I really need. In my 24 I have 3 hermits and 3 different types of snails totally 9. It will depend on your tank.
Since we're on this, are there any snails and hermits that I can put in with a puffer and a snow flake eel? I'm pretty sure the eel is going to eat my blue leg hermits that I have in there now. Would any of those bigger hermits be ok with the eel?