Having just come home from Iraq, I can tell you that in the U.S. Army, the official standard basic issue is 7 (30 rd) magazines per soldier armed with a 5.56mm M16 varient (M4, M16A2, M16A4), for a total of 210 rds per soldier.
However, as others have pointed out, soldiers find a way to "acquire" as many as they think they will need, or can physically carry. I personally never left the FOB (Forward Operating Base) without at least 14 filled Magazines...twice the official basic load. Some were carried in pouches attached to my MOLLE (Load Bearing) gear, while others were carried in a ruck or other bag.
In addition to our personal basic loads, we carried 25-30 additional filled spare magazines in ammo cans on each of our vehicles, and several thousand rounds of additional "loose" ammo (also in cans) for reloading magazines.
Replacement ammo was issued to us in a variety of packaging depending upon what the ASPs (Ammo Supply Points) in our area had available.
In the familiar wooden cases, you almost always found the usual NATO metal or plastic cans, filled with either cardboard boxes of pre-filled 10 rd stripper clips in cloth bandoliers, or separate 20 rd cardboard boxes in bandoliers without strippers.
5.56mm Ball and Tracer was issued separately, and had to be mixed 1-in-5 and loaded into strippers and magazines by hand.
As we always loaded our own magazines, the strippers were preferred, but because of the demand, they were less likely to be issued. Most often we would receive 20 rd boxes that we would then load into re-cycled strippers ourselves for future convienience.
We also carried FRAGs, Colored SMOKE, Thermite, Flash-Bangs, and AT-4s.
Finally, on each of our vehicles we had several thousand rds of spare belted (disintegrating link) ammo for the vehicle mounted crew-served weapons, be it 7.62mm (M240b), .50cal (M2HB), or 40mm (Mk 19).
None of this is unique to Iraq, as this is the way the U.S. Army has done it for all of my 26 years of service, and from what I've read or heard from those who came before me, that's the way it's been done since at least the 2nd World War.
There's nothing glamorous about Warfare, it's an ugly, dirty, miserable, business.
God Bless the USA!
CptA