i would say if you grow vertical you can do 7 plants. some people could do more depending on their style of growing, but imo if they are doing more plants then they will be using a higher wattage. a 600 watt would be better for this set-up but a 400 will work.
1 plant kept short and bushy by lst placed in the center under the bulb
6 plants either taller or raised up around the perimeter, and rotated daily or every other day
the bulb will be lowered down in the middle.