Depends how big you plan to grow it. Basically you'd want the as much of the plant's foliage as you can get within 7" for 42W bulbs. Think about this like coverage. That's about 14" between bulbs. I'd suggest growing a minimum of 4 plants, you can place all four plants around your light source(vertical). And they can grow as tall as you like, assuming you keep adding bulbs every 14".