I have about 5 astrea, 3 Mexican Turbo Snails (large), 25-30 Nass snails, A few Cerith.
I used to have alot more Astrea but they just get turned over and die, the Cerith I thought didn't have this problem but it ends up they do as I have noticed most of them just end up on there back all confused. When I rescue them they are good to go. I lost my original 3 turbos the same way.
If I could keep everything alive I would have the following:
10-20 Astrea
30 Nass (pretty much what I have)
10-15 Cerith
3-4 Turbo Snails (close to what I have)
I also have 2 emerald crabs, 1 short spined urchin, 5-6 Hermits (would like to be able to maintain 10-15 but they don't seem to live long), 1 decorator crab as part of what I consider my quasi clean-up crew to be. Most people stay away from crabs in reef tanks due to there oppurtunistic nature of feeding.