It's really up to you. If you put two plants under the light, you could make two really huge, bushy plants. If you put twelve small plants under the light, then you'll end up with twelve small plants in a SoG (Sea of Green) style grow. A lot of times the number of plants doesn't effect the overall yield nearly as much as your growing style. I personally would probably grow about 4-6 plants and try to keep them in a 3' x 3' or 4' x 4' space (I don't think a 400 w HPS will really adequately cover 5' x 5' without some side lights.