Seriously though, I'd recommend that if you want to naturally prime a keg, use priming sugar or DME - they're WAY cheaper and more easily measured.
And simply hooking up to CO2 and waiting for it to carbonate will be even cheaper and usually slightly faster.
But if you absolutely must use the carb drops (I'm assuming you're using Cooper's), I'd figure about 48 should do it. I typically bottle a sixer out of ever batch, and one drop per bottle has worked well enough for me. Since a typical 5 gallon batch is about 2 cases, I'd figure 2 cases worth of drops should do the trick.