Place a civilian in 3den.
Start the simulation.
Drops everything on the ground
Execute this command :

player addUniform "U_C_Poloshirt_stripped";
player addVest "V_TacVest_blk_POLICE";
player addBackpack "B_AssaultPack_khk";

Now, execute this command :

_class = "10Rnd_762x51_Mag";
_count = 30;
hint format ["%1",player canAdd [_class,_count]];

It will return "False" (So it should means that you can't add 30 mags to the player's inventory)
But if you execute this :

_class = "10Rnd_762x51_Mag";
_count = 30;
for "_i" from 1 to _count do {
    player addMagazine _class;
};

It will add 30 mags successfully

Note : if _count = 20 the command will return true
And 20 is the ammount of mags i can store in my backpack