Like most everyone else, I usually buy in lots of 100. If I'm doing load workup, I'll pull 20-25 casings and use them until they're done. The reason for the 20-25 is that I MOST always shoot 5-shot groups, and usually head out to shoot with 4-5 different powder charges per bullet/primer type.
I continue to run this routine until I get a combo I'm satisfied with. If it's a deer load, then I'll run off a couple of boxes of it. If it's a varmint load, considerably more.
Just the way I do it.....other guys like their own routines too.......