3-4. I'm just running a couple of cheap Vipar 300's in my 2x3. They're doing pretty good for two little lights & 2 plants total but I wouldn't push them beyond 2 plants each, and that's kind of a stretch IMHO. So for a 600 I personally wouldn't push past 3. Of course alot of this is straing dependant. If you're growing short autos then......