If something is the final authority then it better be onboard the airplane in flight, otherwise it's completley and utterly illogical. However Airbus chooses to structure their manual requirments is up to them, and I hope to never have to learn, but that would dictate that the minimum required on board is based on the certification of the aircraft, hence;
The correct answer is 0 or 1. I wonder how you're gonna go about telling me that an aircraft which was certified without a QRH suddenly requires a QRH. As mentioned 2 posts back only the PNF should really need a QRH available to them, don't see a need for 2 onboard in that case.