If you know how large your grow space is (for example, the size of your grow tent), you can easily figure out which grow light(s) will best cover it. The following are the approximate maximum coverage areas for the most common wattages of MH and HPS bulbs:

150 watt: 2 x 2 feet (or 4 square feet)
250 watt: 2.5 x 2.5 feet (or 6.25 square feet)
400 watt: 3 x 3 feet (or 95 square feet)
600 watt: 4 x 4 feet (or 16 square feet)
1000 watt: 5 x 5 feet (or 25 square feet)

From that, you should be able to figure out which size grow light is best for you and how many you need.

All else being equal, 600 watt lights are the most efficient (meaning you get more output per watt of electricity used) and 1000 watt bulbs are second best, so you always want to see if those will work for your space first.

For example, say you have a space that is 10 by 20 feet, or 200 square feet. That is perfectly divisible by 4 (the 150 watt bulb) or 25 (the 1000 watt bulb).

That means, you could light the area with 50 bulbs of 150 watts each (200/4=50) or with 8 bulbs of 1000 watts each (200/25=8). The eight 1000 watt bulb option is far preferable.

That said, you could also grow light the area using 600 watt bulbs.

The area is not perfectly divisible, but 200 divided by 16 gives you 12.5, so you would just use 12 or 13 fixtures and make them fit.