I have been raising red cherry shrimp in a ten gallon with no ill effect. I bet you could probably keep over 50 in there no problem. My shrimp don't eat the moss or plants, this tank has enough algae(to my dismay) that I haven't fed them real food in a couple months and they are still proliferating like crazy. I am using the tank as a grow out tank for my stem plants and riccia(I don't understand why people buy this stuff more than once I wish it would stop growing). Look on planetinvertz.com for a listing of shrimp you can keep together without cross breeding and remember that some shrimp (amano for example) need slightly brackish conditions to breed.
Any of the dwarf shrimp species can be kept and bred in a 10 gal. A few years ago I had a red cherry shrimp (RCS) breeding tank in a 10 gal that kept around 100 adults and another 100-150 shrimplets. They don't eat moss, but they eat the algae and other microorganisms that grow on the moss. What is most important is what kind of water chemistry you have (especially your pH and, to a lesser degree, your water hardness); some shrimp species can only thrive and breed in acidic conditions, others can only live & breed in alkaline conditions. So if you want your shrimp to be successful, it is important that you choose shrimp that match your water.
The chart Ravenous recommended is a good one, but I highly recommend that you NOT buy any shrimp from Planet Invertz, as lots of people have had really bad problems with that company. Instead, I'd recommend AquaBid as a really good source to get shrimp from some fine shrimp breeders.