i currently have 20 plants in a 2x4 tent running scrog and as long as you prune and do some sort of training it really is all up to you. my buddy a few houses down is running 42 plants in a 3.5x3.5 box and he gets around 7 grams per plant and still content. but absolutely make sure you have ample ventilation because hes got 2 600 watters and theyll burn the shit outta the plants if you pack it too tightly.
(edit:theres a lot to be said for very limited veg period (i dont veg. at all.) and cut the undergrowth putting all of the plants energy into the main cola. to explain more, i wouldnt do that with a sativa dom strain but it all kinda comes down to trial and error as well as common sense. a good number of smaller plants, packed closer together with no veg time keeps them short stalky and since theyre going to yield at most 18 grams... you can cut that undergrowth and pack them in there. from my personal years of trial and error i know what strains/phenos can handle no veg as well as i pack my plants 6inches apart. my main point is like what someone else said... you make a general plan, but dont go overboard because if you go less you wont screw up a whole harvest youll just know next time that you can go bigger. seriously, theres nothing anyone can really tell you it really is something you have to experience when it comes to planting density, strains, nutes, etc. i wish you the best of luck.)