Not sure what your asking for in height. If its the res, its more a question of gallons than inches. I am using an 18 gallon rubbermaid ruughneck as my res. Its actually holding 11 gallons of water during the grow. I have holes cut for 3 plants in that res, but only 2 are currently growing.
As far as height for the plants, thats entirely up to you. If you LST and Scrog you can grow in a much more limited height area than if you just want to let them grow, or top them and let them grow. So your height is really up to you.
As for how many plants, If you are going with a nice HID light you should figure on 1 plant per sq ft from what I have read. And a 400W HPS can handle a 4x4 area, so for a 4x6 you would want to look at 600 or 1000w HPS