Depends... what weapon system, lenght of deployment, area, EC, terrain, number of people in team and loadout, secondary weapons in element, is resupply available, distance from QRF forces, ect...
M40 variant, 2 man team spotter/shooter, scout mission in the middle east, secondary is sidearms and spotter with carbine or appropriate. Distance and resupply im unsure of.
Excluding the rounds IN the gun, take 50 rounds ON the gun. I put one of those 10 round elastic buttstock round holders and slip it on so it goes around the scope (if you have a sunshade on it) and the stock. then two boxes (40 rds) in a pouch on the buttstock. Thow an extra box in a thigh rig if you feel like you need it. Butt its all really METT-TC. If you have a feeling its gonna be a hellstorm, bring as much ammo as you feel practical.