Depends on how far apart the cobs are, but 1 per sq ft in that 6-9" area is good at 23w each, assuming you're talking about cxb3590 36v @ 700mA bare cob or 90° reflectors.
But say if you crammed 20 of them in like a single 12"x12" fixture in a 4'x4' tent, you'd want to hang them 24-36" high.
Depends on spacing and area to cover more than individual led wattage.