Depends on how mature the tank is and the amount of algae. For newer tanks less is more. Until the tank can sustain the animals you get, you should start out with about 1-2 per 5 gal
of tank volume and then build as needed. For a 55 gal
, about 10-15 would be good. A mix of different species will also help. Some prefer different areas of the tank. Cerith, nassarius, strombus, nerite and margarita to name a few.
, you will probabley still need to clean the glass weekly, it will just be much less.