For the best yield possible, you'd need a panel for each plant, with proper spacing in between each plant.
That way each panel is side lighting the other plants around it as well as taking care of the one directly beneath it.
BTW, there are much better LED lights out there, that will get you a better yield, and last much longer than the Blackstars.
Area 51 is the best of the bunch and could cover the same grow space with less LED panels needed.