LOTS! J/k...from the PAR the 28 RB leds put out, I would guess 112 total. That would get me two 56 led arrays to cover my 4 ft tank. That's on the high end for sure. I have a SPS dominated reef that requires more light than most tanks i've seen so my #s may be higher. If your trying to = 250w metal halides I would guess 42 LEDs per 24" of coverage.
