The standard Mosquito FB.VI loadout was 150 rounds per gun. This wascontained in two bins of 300 rounds per bin, with two guns feeding out of each bin. So far as I can tell, some author at some point mistook the bins as being one per gun.
Due to the difficulty in finding the ammount of ammo carried by the FB.VI I contacted the RAF Museum. They sent me the info that the standard loadout was 150 rounds per 20mm cannon and 500 rounds per .303 machine gun with an overload of 175 rounds per 20mm cannon and 780 rounds per .303 maching gun.
I knew that Pyro was awear of the standard loadout because that is what he had given the WB FB.VI, but as I had not seen the overload totals anywhere else I forwarded the RAF Museum's data to him. He gave both ammo loadout options in AH.