As I understand it, light intensity increases exponentially as you get closer to the bulb, which makes sense if you think about the way the volume of a cone increases exponentially with height. If one plant is twice as far from the light as another, it will get a quarter of the amount of light. I think that's what Wires means by one light per plant. Keep each plant as close as practicable to a lamp. But if you have one of those big hood things that covers all the plants, then all of the plants can fit under one lamp. The bottom line is, if your plants aren't leggy, your light is probably sufficient (if not 100% optimized), and if the leaves are burning from the heat or light intensity, dial it back a notch. In other words, just pay attention to your plants, and they'll tell you.