I mean, it depends on you. I believe (correct me if I'm wrong) that the light footprint of a 400w is like 3'x3' or 4'x4', I forget.
But either way, the amount of plants you can fit under your lights footprint depends on pot size, veg length, if your running 12/12 from seed, strain, if your doing any kind of training, and what your overall goal is.
If you want an O per plant, your gunna wanna plan for fewer plants vegged longer. if you want the least amount of time seed to harvest, I'd say run a 12/12 from seed. It all depends.
But if your just looking to experiment with sog for the first time, I'd say do 1 plant per sq ft, in 2-3 gallon buckets, give your girls a good two weeks of veg before switching to 12/12 and you should get a good 14ish(+or-) grams per plant with a relatively short grow cycle (seed to harvest I'd say 80-90 days)
Hope I helped or steered you in the right direction