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ALL OF IT
BUY IT
RELOAD IT
STEAL IT
HOARD IT
BURY IT
STASH IT
GET FUCKING AMMO
On a more serious note I wouldn't operate with anything less than 180 rounds on my person. I don't also assume that I'm going to use every last bullet (unless it's a REALLY GOOD day) so you can figure around 1800 rounds of ammo will be good enough for 10 engagements. When you factor in 5.56NATO and .22lr being the most common rounds in most of the US and Canada along with 7.*2x39 you realize just how much ammo you'll be able to scavenge as well as reload if need be.
Within the first few weeks of the large ice house social most of the streets will be littered with brass so get your brass jew on.