All depends on what you want to keep. I called ai direct for my 300, and because I am setting it up for all sps they sugguested 12 units which I thought its gonna be over kill. I ran 3 on my 210 with it hanging 16 inches awl just to test them out, I was pleasantly supprised on the coverage and how the corals are reacting to it. Compare to 3x250 and 4x80w t5 it was definitely dimmer but if I take out the t5ho it's almost identical coverage and brightness. I know 2 on a 48" with mixed reef will do just fine(12"-14" awl) 3-4 on a sps, lps tank(>12" awl) if you need more reference just check out ai's website, they do offer a guide chart for number of units.
3 places to get your AI's
Concept $368 5%tax(AB)
Modular $389 5%tax(AB)
ocean aquatic $377 12%tax(BC)
http://support.aquaillumination.com/...ecommendations