The light doesn't come from the top of the plants it comes from the center. The reason you can have so many plants is the angle at which they reach for the light. The problem with lights is that they don't have enough penetrating power. See, the sun has tremendous penetrating power and it works great in nature, grow lights, not so much.
These systems are designed in such a way that the angle created allows for a lot of plants to "see" the light without shadowing each other. Plants grow one on top of the other, but the light comes from the center, not the top.
These setups are used by professional growers, and I don't mean our type of growing, all of the time. Lots of fruits and veggies that you eat are growin in these things. It's a widely accepted method of indoor growing.