Im just posting to back up what Ink the world said, he nailed your question.It really doesn't matter about plant numbers if thats why your choosing 64 plants, you will yeild the same amount wih 1 plant than with 100 if you use the same light. Also if you get caught 64 plants will get you in some serious trouble.
People aim to get a gram per watt with HID lights good growers can get slightly more,(depends very much on the strain though) so with a 600w you can get 600g if you feed right, your environment is perfect, and have good genetics.
No one will be able to get even 1oz a plant with 64 plants with a 600w. 600grams = 21 ounces, you will NOT get much more than this unless you use more lights.
And the most important thing the genetics, just like I.T.W stated not many people if anyone who knows what there doing use greenhouse seeds, seriously dude use some better genetics, greenhouse is by far some of the worst genetics available.
Also the bigger the roots the better don't try to cram your roots into as small pot/space as possible, this is a common mistake by noobs that think they save space by doing this, but your only fooling yourself and yield will suffer tremendously if there is not enough space for the roots.