I've never used a MH light (strictly cfls to date), so take this with a grain of salt . . .
My grow cab is 22" deep x 30" wide x 48" high. I vegged for 2 weeks and then went to 12/12. I originally had 4 plants going, but had to move one out due to space limitations. It's currently day 34 of flowering, and getting very crowded. Height is not an issue, they're only about 2 feet tall (3 feet if you include the pot), and have slowed down considerably in vertical growth. But horizontal growth (bushyness) has got these 3 plants overlapping eachother and being shoved up against the walls.
Regardless of which light setup you use, I wouldn't attempt to grow more than 2 plants at a time in that space.
