Clean up crews are kind of tank dependant. One person could get by with say 5 snails on a given size tank and another person might need 4X that many for the same size tank. Some general rules you will se will say 1 snail per 2 gal
or even 1 snail per gal
of tank capacity. But ifyou dont ahve problems with alage that many snails will be excess.
I would say if you feel you need more you could slowly build up the snail population. With the hermits make sure you have empty shells of various sizes in the tank so as to help limit the number of snail murders that take place in the tank.