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I guess it depends on how even you really need the light to be. If you're shooting clothing, you probably want everything spot-on from head to toe, but for a portrait, maybe it doesn't matter as much. So, what kind of shoot is it?
Two will guarantee even light, but one can get you extremely close, with just a slight falloff of light (if any). I bet you could get within a third of a stop from head to toe with proper positioning of your light relative to the panel.