I'd easily go to 16, maybe even more - a buckler (or what we might probably use, something like an Indian Dhal) is really sturdy, and while it might not stop your arm from getting broken, the shield itself is all but unbreakable (you might want to go with different values for the shield's own sturdiness and how much damage it absorbs). Swords break a lot more easily than a sturdy disc of metal - that fairly small shield weighs over 2 kg! Also, bucklers are used a bit differently than regular shields - used to cut off angles of attack rather than being used for blocking.
Also, I wouldn't go the typical RPG way for bucklers, where you're allowed to have a weapon in the same hand - yes, there's that buckler&dirk, but it's really not the default way of fighting.
Best of all, bronze is one historical material for Dhals, so it provably works.