Depends.
What's your budget? (As 2jaded pointed out, it can get very expensive.)
How much space do you have?
How big is your backdrop/wall?
How much power can you pull?
Can you hang lights or will they all need floor stands?
Will there ever be more than one subject on camera?
Very basically, what you need is a bright, even wash of light on your green wall. Then, you need proper seperation of the subject(s) from the wall. If they are too close you will get shadows and green spill cast on the subjects by the wall. So, after first lighting the wall, you need -- at a minimum -- a basic three point light package per subject (a key, a fill, and a backlight).
At a minimum, I would say you need two lights for the wall itself, softened either by being fluorescent or by using softboxes. A bigger wall will require more than 2.
Then you'll want the three (minimum) per subject. Bare minimum=5 instruments.