Different folks prefer different things.
I prefer a mix of Astrea and cerith snails. 1 astrea for every two gallons of water and I try to have at least one cerith for every two astreas.
Sooo, about 30-35 astreas and 15-30 ceriths.
If you have a sand bed, then more ceriths.
Also if you have a DSB, I highly recomend the Nassarius snails as well.
I would say 25 or so would be pretty good.
And I, unlike most folks on this board am I big fan of Crabs
I'll probably be banned for this, but I like emerald crabs and micro blue leg hermits.
Nothing keeps valonia algae in check like an emerald crab. And they do a good job on keeping a lot of the macro algae in check. I definately do a lot fewer of these guys though. One or two emeralds will be enough for that size tank, and maybe a 10 or 15 micro blue legs.
My tanks crew is mostly snails, lots of snials. I'm a huge fan of astreas and blue legs.