Most of our work that we did outside the wire never lasted more than 48-72 hours, so my biggest concern for that duration was water.
I would generally wear eight mags on my IBA, one in my M-4, and have another stashed either in a cargo pocket in my pants, or would sit it within easy grabbing reach on the dash of the Humvee. So, generally ten mags.
Also, two mags for my pistol plus the one in it.
At first, ammo was issued to us on stripper clips that were issued to us in ammo cans and we loaded from that. After a couple of patrols and the subsequence de-charging of magazines, we all just started loading loose ammo by hands into our mags.
NRA Life Member
Big Sister: "You should be sponsored by Allen"
Me: "If you can't shoot good, at least look good walking to the firing line."
Big Sister: "Can you not afford a Pelican? Then buy an Allen gun bag."