How many plants under 400 watt hps? - not SOG!

Hi, first time grower here.
I've done plenty of reading, but of course, everything is referring to SOG methods. I'm just doing a straight seed to harvest grow and am trying to get an idea of how many plants, bubblegum and white widow, I can happily put under a 400 watt hps light. I'm growing in soil.
Also: my lamp is a sun system 2, and since the bulb is horizontal, does the light tend to spread out in a rectangle as opposed to a square? I haven't really tried to discern hands on, it's so bright it's hard to tell in a small space.

Thanks and thanks!

four flowering females is a full house....6 is about the max...and that involves a lot of rotation.

you can veg about 12 max