if i was growing in a tent your size, assuming your growing clones, I would do nine plants in one and a half to two gallon pots. try for a sea of green canopy if possible. Keep them short no higher than three feet. I know you said your room is 7 feet tall, but with a 400hps you will only have a small portion of the plant canopy covered, the lower branches won't get enough light to produce the kind of buds you're looking for. So you want to focus the plant's energy on the part of the canopy that gets the most light. The way you do this is by removing the lower branches of the plant. You can squeeze your plants in closer together when you do this and you end up getting a higher quality easier to manicure and harvest alltogether yield. The method of removing lower branches is commonly reffered to as lollipopping. Google it for more info. It also helps with airflow and keeping your soil surface clean which in turn helps combat pests and diseases. Good luck man, its all a learning experience, I'm sure with a little practice and trial and error you will dial in that room in no time!
