you can pull of as many as you can fit in about a three by three area but that pushing it. best shot is 4 or less med to small plants and use the cfls lower on the sides of the plants since low intensity hps don't penetrate that good
I'm running my veg room with one and a pair of CFLs
(take a peak at my journal to see what a 150W can do for veg. Doing OK in my eyes)
I plan on getting a second 150W tomorow and run the two in the flowering room and use a 70W HPS and CFLs for the veg room. (already have the 70W HPS otherwise Id go MH+CFLs)