It depends which method you use to grow. The general rule for best results is 100w per plant. I use a 600w HPS and have done around 8 smaller plants under it or 4 huge ones. Good luck
you can use it for both, just not at the same time. the light cycles are different for vegging and flowering. you can't use it unless you have it really far away for seedlings, as they get scorched really easily under too intense HID (MH or HPS) lighting.
I did my first grow using just my 600w but like many bulbs on the market now they have a little of both light spectrum. They mat stretch a little more than usual during veg but from then on its all good. Good luck