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If you're planning two rows of 4 and 8, you've got 12 total. Try three rows of 3,4,5. Sitting, kneeling, standing. Make sure they're lapped so no one's directly in front of or behind the person in front of them.
You want to have your lens equidistant from front and rear rows to manage depth of field. You want to use one big umbrella above the lens so you're not getting crossed shadows from multiple light sources. I prefer to use a lens equal to the "normal" lens for the camera to minimize distorting the shapes of the faces at the ends.