The standard load for any US soldier is 210 rds of 5.56 mm ball ammunition. When I was in OIF 1 I had the standard 6 mags in my LBV, then I hooked 2 more ALICE mag pouches to my IBA giving me 6 more. The 13th mag was put into the weapon. I also had a mag drop pouch, instead of using a D ring for the empty mags, I had a large pouch connected to my IBA that I could put empty mags into until I had time to reload my mags.
Today, serving in Iraq yet again, I have all MOLLE gear. I have 3 mag pouches on the front of my IOTV (body armor) which is 6 mags, and then another mag pouch just under and to the front of my right arm ( shoot long guns lefty so it's easier to reload from there). In front of that mag pouch is my Beretta 92f. I also have a mag attached to the buttstock of my M4, and finally my Red status mag I keep on the dash board of my MRAP. In addition to this each one of my trucks (4 in one platoon) has no less than 20 extra mags available in a bag inside the truck...just in case. 7 mags is the minimum, after that, it's all about preference.