I grow ten clones a season in a 3 x 3 1/2 closet under a 600w HPS. With that being said, I would grow 6-8 plants under a 400w depending on the size and strain of them. I usually Veg until they are 12" inches tall and well rooted, that way by the end of the cycle they finish at about 3ft tall, yielding roughly 1oz a plant.