As for how we carry it, it depends on the unit. Some units have the Vietnam style three-mag pouches, some units have the 1990's vintage load bearing vest with pouches for six magazines, and a lot of your Regular Army divisions had the MOLLE revolution.
For me, last tour, dealing with ammo carry only, otherwise someone will move it to the gear forum:
MOLLE load bearing vest, reversed to a RACK-style. Four double mag pouches. MOLLE six-mag bandoleer in my assault pack, also full. One more mag in a Tac Tailor stock pouch.
Never used more than four or five worth the whole war. My platoon just seemed not to be where the shooting was at the time.
This time, I'm not walking so I rigged a little differently. SOTech Hellcat chest rig, six mags, with a Tac Tailor triple mag pouch added. One more mag in a TT stock pouch. Six more mags will be in the MOLLE bandoleer in my truck. First four M203 rounds in a TT pouch, second four in loops on the outside of a custom 2-quart canteen carrier from what's now BDS Tactical.
Since we practice aimed fire, and have nice things like Aimpoints and ACOGs to help us place our shots, ammo consumption goes down. Even my ten mags wouldn't last long if I treated my M4 like a 5.56mm garden hose.