The thing is when you reach a 'max' the shrimp's breeding will slow down. At one time I had over 200 in a 3 gallon. About 30 were adults the rest were babies, juvies and subadults. And they weren't climbing on top of each other either. And they were still breeding. I've got probably 100-200 in a 10 gallon right now, about 30-40 of those are adults. Still have tons of room. And of course how the tank is set up can utilize more room. Shrimp typically stay on the bottom of the tank, so if you create a moss wall or something going up the back up the tank you actually give them more usable space.
In other words there's not really a 'set in stone' # for max capacity. If you want a very 'basic' rule of thumbs it's 10 per gallon. I'd honestly bump that up to more like 15-20 per gallon in my experience.