Whether it is a lot of light or not depends on what the plasmas will cover. I really know nothing about plasma lights--could you give us a link or some info to the ones you are using? A 600W in a 5 x 5 space is underlit. It does not matter that it is directly over the table, it produces 90,000 lumens and those 90K of lumens do and will disburse throughout the entire space. So depending on exactly how much the plasmas will cover, you may still be underlit, or just at minimum lighting.
How many plants you put into a given area depends on how you want to grow. It is also somewhat dependent on the strain you are growing, too. Hush has given you 2 examples of how different plants react to different training methods. Some take well to scrog, some not so much, some take to SOG, some not so much. Unless you are doing a scrog, I would remove the screen as it can be an encumbrance if you need to get to plants on the interior of the screen.
Are these fem seeds?