I run four plants in 5Gal buckets, soil. By the time they get big the tent (4'x4'x7' also) is pretty full, and if you're going to lst or anything you'll need room for that too.
A 5Gal bucket is typically a foot in diameter so it's easy to think "Hey, I can get 16 5Gals in there". And you can, they just can't get more than about 12" wide. Getting to the middle ones could be a pita, even w/ side windows. And even lighting is difficult, so you might have to rotate plants around.
You don't want to pack them in too tight, remember you'll need good air flow & circulation especially in a small tent, no matter what style you use.
That's been my experience so far, using 5g pails of dirt. As many have said, it all depends on how you want to do it. In other words, about a pound.
G/L, like everything thing else related to this hobby, there's never a single answer, and the answers you do get might be 180 degrees apart. And they might all be correct, depending on who/what/when/where/why.
Occam's razor is useful until you get your chops down.