400 watt light will cover a 3 foot X 4 foot space....or 12 square feet. ( keeping plant tips about 18 inches away from light)
...if your using 5 gallon buckets, like what paint comes in, you can fit about 4-8 (my opinion) plants depending on how far away you keep your lights ( close light = more bud!)....and that's pushing it. Good luck.