I hope that you can get away with 5 or 6, but if you really want the higher light demanding corals, then I would plan on twice that many down the road. Radions are not that high light fixtures to being with and being point source, they create a lot of shadows - this is OK when the corals are small, but as they grow they get really pale on the underside and start to die from the base up as they start to turn into colonies... so folks have to add more.
Some wider panels like Reefbreeders, Acroptics or Pacific Sun really help with shadowing, but people complain about the light blending on those. There is no "perfect" LED that does both.
I am not saying that you cannot do Ok with them, just have a plan for when 5 or 6 will not nearly be enough. If you don't think that you can swing a dozen of them, then you might want to consider something else - this is a lot of money not to understand that it might not be enough for you down the road.
FWIW - Dr. Joshi needed 8 Radions on a 6 foot tank to replace 3 MHs that he ran before. He said at MACNA that it still probably was not totally equal in output but was the same in wattage.
