6 plants would be safe and 9 would be more of a see of green. Just make sure you have plenty of air flow and good light. Watch the temperature of the cab and measure it under the lights at the top of your plants and you should be fine.

The size one plant takes up depends on many factors not posted here. In a small cab one plant can take up 1 sqft or better if you keep it small. Average is more like 30" square for a 36" - 40" tall plant. The taller the plant is the wider it will grow. Now all that is assuming you are not cropping or bending anything on the plant but fan leafs. Then it's totally different.

Happy growing