if you have not purchased the lighting yet, and you want to flower 20 plants in a scrog or SOG then I would do as haze suggested and go with 3 600w lights.
For a decent LED you are talking $400 each. search LEDgirl and check her journal. She seems to be pretty knowledgeable on the subject.
Personally I would go with three 600w hung vertically surrounded by plants. Stadium style.