gotta be Mike V
Mike V had boards from at least 12:
Powell, World Industries, New Deal, TV, Transit, Black Label, Vallely Skateboards, Element, By the Sword, 1031 (guest board), Elephant, Street Plant
But - he owned or was part owner in TV, Vallely, By the Sword, Elephant and Street Plant. If you remove the self sponsors and guest board he's down to 6.
Grosso had quite a few if you count his Am sponsors and the various Black Label sub-labels.
Variflex (Am), Powell (Am), Schmitt Stix, Santa Cruz, Lucero Ltd, Black Label, Red Cross, Emergency, Anti Hero