A 400w HPS will cover an area of about 4 x 4 foot. So you should be good for anything between 6-9 plants under there depending on variety how tall they get how much side branching etc.
id say get another HPS set up maybe a 600 or 1000 and use that for flowering and veg the others ones under the 400 and when they are ready to flower veg more, so you can get a constant harvest. Good Luck Bro!