dont look at it as how many plants under that light, think of it as how many plants can you fit in a certain area....a 400w hps will cover 8sq feet at 50w per sq foot which is optimal...so as many plants as you can pack into 8sq feet...right now i have 17 1 gallon bags in 5 sq feet under a 250w Mh...the babies are only about 11 days old...im not sure if im goin to upgrade to a bigger tent and just keep the smaller one as a veg chamber of if im just goin to upgrade my lights, keep them in the small tent and lollipop them