Snails in
SW aquaria are nothing like
FW tanks. They are no where near as prolific and often do not bread at all. Many of the egg masses laid never get properly fertilized or are eaten by other scavengers. If your cocerned about the tank being overrun by snails, don't be. It will rarely, if ever happen
That said, snails will indeed be your best defence against aglaes as well as detritus. Here are a few good suggestions.
Nassarius great for detritus and the sand bed.
Nerite especially good at film algaes..
Cerith cleans both the glass and the sand bed..
Bumblebees cleans up the uneaten food bits as well as detritus.
Margarita another excellent algae eater..
The amounts of each will vary on the types of algae problems or detritus as well as fish types you may keep. The general rule is 1 snail to every 1-1½
gal of tank water volume. In the beginning though I would start with about half that and build as needed based on the natural food sources available to them.
Cheers
Steve