Post 100!
I'm no snail expert, and there is no exact science to how many to have. It all depends on the amount of food you have in your tank. One person may have a huge hair algea problem, and another have a diantom problem. Both of these can call for different kinds of clean up crew. My suggestion is to have a variety of several types. Astrea, turbos, nass, cerith, ect. Personally I would think 10 would be real good for your sand bed. Most people get nass snails to their sand bed top layer is worked. The only other suggestion I would have is if you dont have much algea, stop cleaning the back and one side of your tank until you can determine with a reasonable accurancy that you have enough for them to eat. Otherwise, you'll have some dead snails, which equates to a few ugly shells. (nass snails are not that attractive). Another piece of advice is to pay attention to the latin name, some places advertise nass snails but dont really ship them. Nassarius Vibex is a latin name for them.
good luck
To answer your question on quantity. I think 50 is way to many. I plan to add them all though and see what happens.
