First make a copy of your dayz_code.pbo file in case anything goes wrong, next open dayz_code.pbo and look for the files in the picture attached, open them and hit ctrl + f and type the name of whatever vehicle you want. Look for this line:
transportMaxWeapons = 5; transportMaxMagazines = 10; transportmaxbackpacks = 2;
change those values to whatever you want. save it and copy the edited pbo where you normally would in your ArmA 2 OA folder.