Tons. Hard to measure.
Honestly, buying 1000 (especially if it is a BOGO) and keeping them in a 10gal with a sponge filter and some macroalgae (Ulva for diet, chaeto for housing) you will probably never run out. Keep two balls of chaeto, net one out and drop it in your cuttle nursery, and rotate out every couple days. The reproductive rate of the amphipods is extremely high.
Our seahorse tank has a large amount of rock and macros in the refugium (along with good lighting). After the tank initially cycled I added some snails, algae and 2000 r2g pods and two years later and I still have yet to repopulate or add any other food for the seahorses - which are fatter and more colorful than any of the display horses at a LFS. I just mention this to give you an idea of how they breed and maintain populations.
Another observation is that the amphipods seem to love cyclopeeze. I feed them regularly in hopes it boosts their nutritional value even higher when the young cuttles feed.
I'm currently getting a new system setup - two identical tanks using the same sump. One will feed the hatchlings mysis and the other amphipods, so I can do better growth and health comparisons. Again my personal experience leads me to believe the cuttles actually grow better on a diet of amphipods but I think this experiment will help determine that a bit better!