Here is some info I found on
http://www.ehow.com/how_5302582_boos...ic-snails.html :
"1 - You can crush up eggshells and tie them into a mesh filter media bag and hide the filter bag behind a plant or decoration in the back of your aquarium where the eggshells will slowly dissolve adding another source of calcium into the water.
2 - You can crush eggshells up into a fine powder. Add the crushed egg shells into a batch of home made sinking foods."
It seems like step 1 implies that there's no magic number - maybe just add them as they dissolve? I'm not sure, but I thought this at least had some good advice for how to add the shell once you figure out how much.
*You can also blanch veggies that are high in calcium and your shrimp will get calcium through eating a tasty snack.