I'd say every answer here is correct already, but in theory, you can grow as many plants as you can fit containers in that space.
Many factors though... since you are growing from seed you will need bigger pots versus growing from clones. And with a 400w you can't let them get too tall, so topping/FIM/LST/SCROG are things to look into (Scrog only if you have feminized seeds).
Having a separate veg/clone/mother chamber would obviously be a huge plus (doesn't need to be huge, check out the bonsai moms link below). You could veg out one of each strain, do some training (LST) so you'll have a lot of clones (
Bonsai Mums: A Guide - Grasscity.com Forums), and put the rooted clones into 3 gal containers filling out the space. Veg for a week then flip. See how they grow and make adjustments from there. Some kind of screen or netting will help keep an even canopy. If it gets too crowded you won't feel bad taking out a couple runts, versus taking out a 3 footer or dealing with shading and mold issues.
^^That's something I was planning to try (if I ever got over having such a high plant count).