I too was using a 400w HPS to flower 7 ladies and it's not sufficient, so I changed to a 1000w. I would say 6 is the absolute max you'd want to try.. And by doing 6, you would only want to veg until plants are 12" in height. . . Even if you LST, shoots will grow and overwhelm your space horizontally. More so for me because I have white widow... (big plant) lolz