If you're talking about cherry/crystal shrimp, there basically isn't a maximum amount you can buy at the start. Get as many as you can afford, I usually start with 6 (expensive shrimp) - 30 (very cheap shrimp), that was for a 23L. They will breed into the right number for the tank. The skies the limit for shrimp!
When I broke down a 30 litre nano many years ago I found over 200 hundred cherry shrimp in there and I don't think that's anywhere near a record from some of the tanks I've seen. They were very happy and I only started with 5-6 that I caught from another tank.
Cherry Shrimps will breed like rabbits and after a while, you will have as much as the tank can sustain... Similar to snails... This number can fluctuate depending on the available food sources in the tank.
For calculating fish size and numbers we only have commercially influenced guidelines such as X cm fish per litre or X number per litre. Personally, I find those formulas as said rather commercially influenced more helping the trade than the fish in question.
This comes down to personal philosophy and perception, and or maybe from experience.