Depends on their strain, how big you intend to grow them, how you manage the lights....
Basically 400w HPS covers 3x3 feet nicely, up to 5x5 supplemental. 1000W can do 5'x5' great, and up to 8'x8' supplemental.
You can fit 1 plant per square foot and form a decent canopy. Bigger plants require more room or training.