how many plants can i grow under my 600 watt hps ??????

I usually put between 13 and 16 plant's in my 4x4 tent. Personally i find you can go up to a plant per foot for max efficiency. I'm running a yieldmaster 2 classic with a 600 watt digital ballast. I usually get between a pound and a quarter to a pound and a half per grow.

Also i am only on my third grow so i consider myself a noob to this whole thing yet still get great results. also start flower at 12 to 14 inches.