Hundreds - depending on experience. A good rule of thumb is 10 per gallon unless you have the right setup and know what you are doing. Some breeders have 100 per gallon in their tanks...
I think if you know what you are doing enough to get them to breed that much, you're fine having pretty much like, as many as will breed in your tank. There's no real issue with "too many shrimp" in a certain size tank like with parameters or anything like there is with fish, I don't think--correct me if I'm wrong.
Quick question too, are amano shrimp good shrimp to put in community tanks?
Amano shrimp are probably the best shrimp for a community tank because they're too big to fit in a lot of fishes' mouths, whereas dwarf shrimp (RCS, CRS and variants) are smaller and are often food (especially the babies--literally anything will eat the babies. Amanos can't hatch in freshwater, so no babies from them). Still, if you have large/aggressive fish your amanos might end up as a snack, but mileage varies. People have had success with all kinds of shrimp in all kinds of community tanks, though--the trick is planting heavily enough to give them lots of hiding spaces so they don't get munched.