At least 4 at that length, even that probably wouldnt be enough. To be honest, with a 5 foot tank thats 2 foot deep, I'd go for metal halide lighting. FOr some perspective, I have a 3'x1'x2' (LWH) tank with 2 96watt power compact bulbs on it and I have some difficulty growing high-light corals. Does your tank have 1 or two braces across the top as that will greatly affect your lighting decision.