I am using one 40 optic over my frag tank, which is in 1/3 of a 29 gallon that is full to within 2" of the top. I recently had to lower my frag rack and raise the bulb a bit. I have monti cap, monti digita and zoas in the frag tank.
It really depends on what corals you are going to keep. Over the 29 biocube, oyu might need multiple bulbs or one with a wider optic.