There are some packages on this site that can give you a clue, but they span a wide range so it's hard to tell. I've heard 1 per 2 gallons. What i've been doing is buying about 5/10 crabs/snails each week or 2. I'll be doing this until everything seems clean. I like this approach because I am adding crabs and snails as my bioload grows so there will be a larger cleanup crew as it's needed. I would start out w/around 40 maybe and just slowly add more.