I will echo what others have said on size, go bigger than you think. In my experience, the number of guns a safe is supposed to hold usually equates to slender, bolt action rifles with no optics. If his rifles are AR platform, have optics, or a combination, that space fills up very quickly.
I have a "16 gun" cabinet. one side is completely full with three ARs (two AR10s and one AR15) with optics. The other side is full with a lot less that 8 long guns.