how many plants for a 6' X 4' X 2'

Is your box 6' tall?

its actually like 5.5' tall but yes.

Depends on how u wanna do it. If there is no training involved like LST or SCrog then i would only do 2 and grow them about 3 feet tall. Thats what i did, however if u Scrog u can get 3 in there or sog u can probably pull 4 off. A grow space of 2x4 isnt very much room, trust me my cabinet is a 2x4x6 and i can barely squeeze it anymore than 4 at one time. Your best bet would be to stay with 2 plants and grow them large. If you got the right light setup u can squeeze a half pound or more if you know what your doing. With only 225w hps i would do 2 or 3 with a scrog technique. You would get amazing yields and still maintain a reasonable air space in the cab or tent.

I typically grow organically in a 2.5'(W) x 6' (L) x 7' (T) closet w/ two 600w lamps. Last go round I ran 20 White Widows in Scrog and ended up with a cured K.
I don't think that 10 is out of the question with your grow space.
Your turn.

The rule of thumb is 1 per square foot of space. However I don't see a point in doing so many in a small space unless your trying to do a few different varieties, and even then you will be hard pressed to keep every strain the same exact height and width. I'm not saying you can't stuff that many in there... I just personally wouldnt. You could easily do just 2 relatively like strains and grow them pretty big. I got 7 ounces off my last grow in my cab with a power kush and vanilla kush and that was with humidity at about 60 all the time and temps around 82. However you do it I'm sure you will make it work for you. Goodluck.

At one per square foot, doing a scrog style grow you can get 8 plants in there. It would be tighter than Dick's hat band, but doable.