A buddy of mine told me I could grow 1 plant in a space about 1-1/2 'x 1-1/2' x 5', This seems sort of small to me, I plan on growing 12 Medical plants So what would be the minimum amount of space need to grow this many plants?
It depends on the size of the plants. I've seen plants that basically took up just 5" diameter and a foot of vertical space at harvest time. Personally, I always plan for a square foot of floor space per plant and get info on how tall each strain is likely to become. You should probably adjust your number of plants so that they fit into a square in order to distribute light evenly over all plants. This would be either a 3x3 pattern or a 4x4 pattern. Using about a square foot per plant with a few spare inches, you could use a 600w for 3x3 or a 750-800w for 4x4. Sometimes a circular pattern can be used.