it depends on how many plants you have and how much room you have to work with but use as many as you can get and one is no where near good enough i would use 5 lights at the least
you can get by with 1 plant, if you have plenty of reflector, and plant is kept under 3 feet
i'm flowering with 78w right now, grow is in my sig
i've flowered once before with this setup, so it can be done