Wilt and Talley have covered the one-light argument, and I agree. I have seven studio lights, and typically use one—or a single Speedlite. The functions of key, fill, hair, rim and b/g can be handling with one light (even the sun) if you have a good mix of modifiers and the means to deploy them. By modifier, I refer to various materials on panels, and deployment will require stands and grips. The easy (but not really satisfactory) workaround is more active sources, where those sources do not comprise a believable lighting scheme: hard-light rim is my pet peeve. If you get two more lights, even Speedlites, you still need to think about placement and modifiers, otherwise you just have three directional lights spraying photons and shadows everywhere.
Canon, Nikon, Contax, Leica, Sony, Profoto.