Standard load out, as in loaded mags on your LBE/armor/plate carrier, for infantry units was 360 rounds which breaks down to 12 mags. Some of the SF guys carry less than that on their plate carriers and keep the rest in a backpack or something but that only really works for them because they're so good and accurate that they can accomplish more with less rounds than average line unit grunts.

Those extra rounds come in handy if you have to break contact or something.