4x4 tray... you need to start with known females in a tray like that unless you are gouing to containerize your media and thus be able to remove males without damaginv the roots of the other palnts.... your hydroton bed is uneccessary.. individual pots/ blocks that can be moved around are IMHO the only way to go if you are dealing with unsexed plants.
But if you werre starting with female clones I would stick either 9 or 16 plants in that space depending upon the size they are going to be when you switch the lights to 12-12