Welcome to 420mag! A 400 is good for one big or two smaller plants. Or you can go smaller plants then that and do more. Think of it not as a per plant thing but instead an area thing. 400w HPS is good for about 2.5ft to 3ft square max.
Thats a lot. I don.t know how you will get that done unless they stay very small. Very small. You would have to keep them very trimmed back to do so. For comparison I have 12 plants under 2 600s, and 3 260 true watt LEDs and I am cramped for space.
You can do it just very small plants in small containers. Which can be hard to do as they require a lot more care. Ricky Bobby has a grow you can check out. He uses 2 liter bottles for pots and fits in the numbers you are looking for.
I believe Shiggityflip has given you some very solid advice. I have seen Ricky Bobbys grow and what he does is really amazing. I have also seen Shiggityflips garden and can not possibly imagine how he crams all those plants into such a small area, I have 3 plants in a 5x10 and feel cramped trying to do daily maintenance.