(U.S. Army) FM 3-22.3 states 36 rounds
600 to 800 rounds
the army base
Combat Basic Load for the M16 and M4 is 210 rounds. As the M4A1 is typically issued only to select units (Army Rangers, AF Para Rescue, etc.), personnel assigned this weapon would be likely to carry more than the basic load.
They don't carry ammo in boxes. Basic load depends on the weapon system. 210 rounds for the M4 or M16, 600 - 800 rounds for the M249, 800 - 1000 rounds for the M240.
The M249 Squad Automatic Weapon fires the same 5.56x45mm cartridge used by the M16 and M4.
The maximum effective range of the M249 light machine gun is 1,000 yards (910 meters).
In some countries which field it, it's operated as a crew served weapon. However, in the US military, it is fielded as an individual weapon.
buckle means bending
CLP
FN Herstal
They could be assigned an M16A2, M16A4, M4, M249, M240B, they could be put on an M2... depends on rank, position within their platoon, etc.
In short the M249 is effective up to 1000 meters to cover an area target which the gun is designed to do.