150-175 watt lights are primarily used for seedlings or cuttings, though they can be used for growing plants to maturity if the plants are kept short. 250 watt systems are good for areas up to 2.5' square at the most, as long as the plants don't get taller than about 2'. 400 watt systems cover a primary area of about 3' x 3', or up to 4' x 4' max. 600 watt systems cover a primary area of about 4' x 4', or up to 5' x 5' max. 1000 watt systems cover from 4' x 6' up to 5' x 7'. The taller you plan to grow your plants, the higher the wattage needed. This is because the light intensity diminishes by 50% for every foot you move away from the bulb. So, if your plants reach 4' tall, then the leaves at the bottom of the plant are receiving only around 12% of the light that the top of the plant is getting!
Depending on the grow method you plan to use also will determin how many plants. For example, when I do my SOG grow method (different than others) in rockwool cubes I can grow around 25-50 plants. But if I grow using the standard grow methods I can grow about 9-12 plants.
Hope this was some help. Oh, and by the way i'm supposed to send all newbies here --->
How To Grow Marijuana Good luck with your grow!