Eight, if you plan on growing them out to maturity. In my experience, each plant should have a 2' x 2' square to spread her wings. If you LST, you will need more horizontal space per plant. Also, in my experience, you can expect trained plants to stay under 4' tall, so you can add an upper shelf for starts and juveniles, or just storing stuff.
The second shelf should be adjustable, in case you want to go double-decker with LST plants, keeping them low and wide. Two 36" chambers.