What does "Number of subs" in the menu on the subwoofer mean?
This setting is only enabled if you have chosen Reference level On, in the level menu. This setting will enable 1, 2, or 4 subwoofers to have reference level in room, for ease of adjustment from an AVR or similar source. It will decrease the output sensitivity of each subwoofer to come as close as possible for the sum of all subwoofers to reference level for a single subwoofer used alone
So, basically, it is only there to help you get the measurements done without having Audyssey complaining about exceeding the scale!