There was a recent thread about this:
Thread 'How Many Shrimp?'
How Many Shrimp?
The key is just to let them self-regulate by not continually increasing the food. When mine were in small dedicated tanks I would just feed them a pellet or two (or a frozen baby spinach ball) once a week, always have a degrading catappa leaf in the tank, and let them scavenge the rest of the time.