It really depends on your comfort level. When I first started I tried 3 and found it to be very easy and went to 4. I know what you're thinking, why not just go to 5? Well the monthly fees DO add up. That also factored into my decision to 4 box. Plus I only use my quadcore to run all 4 instances of wow. I think if you really want to 5 box, you ideally should go with atleast 2 machines (if not more).

Dual boxing should be fine on a single machine however I think you'll find that you will want more. So go with as many as you can handle and afford, and as many as your hardware will support.