"How many plants can i grow under 2 600w lights?" That's prety vauge. The answer is between 1 and 100. Personaly I would use the 2-600W lights to illuminate a 3'x6' (4'x8' starts to stretch it for a 600W) grow space (the room is bigger) and because I have to keep within limits I'd put either 3 or 6 plants in each 3x3 area. At the moment I am growing plants under a screen SCROG to keep the canopy nice and even and full. If your not concerned about plant count, I'd add more plants and keep them smaller, drop the screen, trim the side branches and grow a buch of top colas.