the answer is how many plants you can put in a 17ft square area, depends on how big they are and what size containers there in.it could be a 100 tiny plants in small containers or a couple big plants in big containers depends on your preference.as for how far above the plants should they be hung it depends on if there are cooled or not but a good rule of thumb is to place your hand at top of plants and if there is no discomfort your good, usually u can play with it a bit tell you think you got it where you want it.