i would put 6 400w HPS's over a 6' x 9' area,fill the area with 136 clones in3/4 gal pots and expect to yield at least 900 grams off of plants that are no more than 24 inches tall.Iwould expect to increse yield 25% to 50% on the next run as i tweak it out . inexperienced growers may yield 100 grams or less in the same area.i would need a 4'x4' mother room/area , a 2'x4' clone chamber and room to move around all this stuff .it is A LOT of work to keep this many plants happy and you need to ask yourself "do i need this much pot at one time?"if your grow is for personal use than may i suggest that you answer NO?Grow for quality not yield. one 400w could yield as much as 200 grams ever 8 weeks and provide one person with meds, two 400w HPS's 3 to 4 people. Remember, as your garden grows in size so does the chance that someone will want to take it away from you while at the same time it becomes harder and harder to hide.Be careful, start small .