it depends on your definition of "good airflow". If you want a system that leaves no dust inside, I would try for around 5-6 case fans, three on the front pointing inwards, and two on the back facing outwards. If you just want straight cooling of a light build, maybe 3 or 4 should be good, possibly less. For anything high-end, I would probably just fill up the available fan slots on your case, but it depends on your system specs and what you're doing with it and your intentions with airflow.