Depending on the mission each soldier may carry a couple of hundred rounds of 7.62 mm for the crew-served weapons and maybe a mortar round or disposable rocket launcher. It depends. A Basic Combat load for the primary weapon is 210 rounds of 5.56 mm. Usually carried in seven 30 round magazines.
But Basic Combat Load depends on several factors. What military, what caliber round, what role in a section, resupply or not, mounted vs dismounted ops. Reconnaissance or direct action. Regardless, it is encouraged for modern infantry solider to carry 5–7 full magazines for his primary weapon and 3–4 for the secondary weapon.
Basic Combat Load
The basic combat load for a modern infantry soldier who might reasonably expect to be aggressively engaging the enemy about six magazines plus one loaded in the weapon is fairly normal but that could easily double depending on what other equipment needed to be carried.
Aside from that, the answer is it depends. Some “front line” combat soldiers won’t carry any magazines on their person, since they’re busy operating tanks, howitzers, or aircraft. Some will carry a lot more than the minimum seven rifle magazines, as well as pistol magazines if they carry a sidearm. And even some infantry personnel have a primary weapon that doesn’t use magazines at all, so machine gunners, for example, won’t carry any.
It all depends on the situation, and most warfighters whose primary weapon is an assault rifle will carry more than seven, if not on their immediate person at least in a pack or in a nearby vehicle, but the basic combat load is one in the rifle and six more at the ready.
Basic Combat Load for SOF operators
Special Operations Forces operators have their own needs, depending on the type of mission, they form their own basic combat load. It does not differ much from the regular army, except in maybe some specialized tools and equipment they use.
Even though they can hold up to 14 Mags, most operators use the sides to carry a sidearm holster, grenades, fast-rope gloves, IFAK (Individual First Aid Kit), etc. It’s not all bullets, we have other needs, too.
Gear carrying systems
The Basic Combat Load recommendation is what most load carrying systems are designed to carry and is usually enough to carry out a platoon level assault, especially as riflemen will also usually be carrying rounds for mortars/40mm grenade launchers, section support weapons, and grenades.
Magazines do represent a certain amount of additional weight and bulk and you get to a point where it is easier to reload magazines from packaged ammunition than to carry more. Even with loose rounds, an experienced soldier can reload a magazine in under a minute and with clipped ammunition and speed loaders this gets a lot faster.
Bear in mind also that in a sustained firefight riflemen won’t be firing rounds as fast as possible and there is a limit to how much weight you can physically carry and still be effective. Equally, in that sort of situation, you are likely to be relying more on section and platoon support weapons than a rifle to achieve a weight of fire.
Many combat soldiers won’t normally expect to use their personal weapons at all, and if you are operating vehicles or crew-served weapons that more than a few magazines for emergency personal defense is going to be a major encumbrance.
Equally, commanders will rotate individual sections/squads in an engagement to give them the opportunity to rearm and generally sort themselves out and will plan and organize an assault or defense such that logistics are properly taken care of.
Even at the section level, it’s always possible to put one or two soldiers on redistribution of ammunition duty when necessary and in a defensive or rolling assault action a platoon might well have a half or whole section in reserve refilling and redistributing magazines and other essentials.
Different armies – different rules
The US military carries 7 magazines (30 rounds per mag) for their M4 as the standard. The Austrian military tends to carry 5 magazines for their Steyr Aug and the German military also carries 5 mags for their Heckler and Koch G36.