Currently the scripting command addMagazineCargo only supports a number of magazines instead of a bullet count.
A scripting command in Arma 3 Similar to http://resources.bisimulations.com/wiki/addMagazineCargoEx would be nice.
So you can preform commands like...
(unitVest player) addMagazineCargoEx ["MagazineClassName","BulletCount"]
to add magazine with a certain bullet count.
You can add magazines with a bullet count with addMagazine (Array) however you cannot control where the magazine will end up, or add magazines with bullet count to storage objects.