With Adult RCS I've read you can have up to 10 per gallon ... Though 4-5 would give them much more room. With baby shrimp it gets more confusing ... Their so small with such small bio loads they really don't affect your water quality. The big trick is making sure there's enough food for the babies. Fortunately, they feed on the same stuff the adults do. Also leaf litters really help with both food and shelter for baby shrimp ... Though in an RCS only tank the babies are safe.
Honestly, because of how fast they breed, start with just 5-6 in a 5 gal. Figure 3-4 females the rest males and you'll be bound to have baby shrimp in no time.