I have read 1 per every 2 gallons.
Not sure if that is realy a good number.
Moving them around to different takes would work.
I bought 6 for my 55gal, that went through a fishless cycle, I left the algea in there for the snails to clean up. They did do a great job, only a small amount of algea to be seen.
I started putting some Plaster of Paris Pucks in there, that I made for my mystery snails, also some zuchinni(sorry for spelling). I have seen them eating some of both. You may want to try supplementing there diet with other foods, if they have cleared the algea. From what I understand it can be hit or miss, some will only eat algea.