the effect surface area for a 500Watt lamp is about 1.0 m2, so it kinda depends on how much light you want your plants to get and how big you want them to grow. but I think you could use a 500W for maybe 6 plants, but I would recommend to get a lamp for every 4 plants. It also depends on the area of your growth, if you grow in a 2m2 box I would get 2x 400 Watt lights. If you grow in a large area then a lot of the light can escape so the amount to reach the plant would be less. I would go for about 4 under a 500W