I find it hard to pick one answer...
...i think i got 5 mags in most of my loadouts, but how many i actually use depends ALOT on playmode, map, lag and skill of the enemy.
With my sig loadout (sig, scope, 5xmags, 2xnades, knife) i default to burst fire at roundstart and often suppres corners that i am expecting heads to pop out from.
With higher skilled enemies and/or light lag, switching to autofire becomes nessesary, because bursting at people don't work when they are not there by the time the packets reach the server - wheres as auto fire allows a kill because he will run into the fire himself. (which is why the drum AK rules so much some maps, since it produce lag and takes advantage of it with big clip and autofire.).
I don't run dry very often with 5 mags, neither DE og Sig loadout, but some maps just means hands you more "tough firelines" or more moving targets that others... all resulting in wasted ammo.
...but since most my loadouts contain one weapon only be it a pistol or a gun +2-5 nades (often 5 in light pistol loadouts) the extra ammo rarely hinder me.