If you want a short veg. period you would just need more plants to fill your light's footprint. It also depends what intensity light you're vegging under. I flower 4 plants per 600 watt light after a 6-8 week vegging period but I also veg. under low intensity fluorescent shop lights.
With a full 2 week veg. period under hid's (if we're talking healthy well rooted clones), I would think 9 should cut it. A good rule of thumb, regardless of how many plants you use, is to flip once 2/3 of the light-print is filled.