I've assembled heaps of the buggers but can't remember. A good place to check is just google "bretonnian knight sprue" and see for yourself.
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I count 8 regular knight of the realm heads, 4 helmeted knight errant heads and 3 non-helmeted knight errant heads (one of the helmets near the non-helmeted isn't actually a head, it's an empty helmet).
So you get 15 heads per sprue, and you get 2 sprues in a box, so 30 heads in a box. If you assemble them all as knight s of the realm, you can construct them all with a unique head and have 1 spare of each of the heads, and if you construct them as knights errant, at least 1 model will have a duplicate head, and you'll have 6 left over heads.
So lots and lots of heads