I am having a new tank built 36" long x 18" wide x 18" tall (inches) how many T-247B will be required?"
I didn't want to potentially reduce the answers you might get, so I'll do it by PM. One could be enough if you can hang it 18" or more above the water and are only doing easier corals (due to low PAR from lots of light spread). If you do two, you can keep the most light loving critter you can find! At 6" to 10" off the water they cover about a 2'x2' footprint. I use one over my 2'x2' frag tank.