12 gauge: 300 or so I believe. I stack my birdshot into a 50 can, and I think I got about 300 in there. I know you can only fit 175 in boxes, but when stacked, you can really get them in there. May be off, been a while since I counted... 300 SOUNDS like a lot, but it's the number that I've got in my head, so... that's what I've got.
9mm: 2000-2200. I know I've dumped 2k into a 50 can, and there was just a bit of room, if I remember right.
.45 ACP: Little under 2000
5.56: 1000
No idea on the .44 Magnums. They're about 25% longer than .45 ACP, so I'll guess that you can fit 25% less...