It depends on how your grow them.

SOG you can use one 1000w HID light on a track mover and grow up to 40 of them.

If you have not purchased the light then you may want to think about 600w digital ballast. You can run either bulb with them and they run cheaper (in power cost) then standard HID transformers and run way cooler then standard transformers.

You need to know how you are going to grow then before you can think about light layout. You grow area will also have a say in how you grow and where you place your lights as well.