for a tank this big you really can't skimp on any of your equipment, lighting, circulation, etc. I would say start with at least 4 but I personally wouldn't have fewer than 6 MP40w's or even 8 for awesome flow on that tank.
With 8 pumps total I'd have 2 on each side and 4 along the back wall, with 6 total I'd have 1 on each side and 4 along the back wall.
-Tim