Also in consideration of number of runways is the weather in the area. My base of Phoenix, AZ for example is the busiest 2 runway airport in the world, but this is due to our great weather year round. If the same airport were in Pennsylvania or Seattle, there is no way it could support so many aircraft.
Here, aircraft are separated visually, usually 3 miles apart. In bad weather it is increased to a minimum of five miles. Also if the runways are too close together (3000 feet?) you cannot use both of them at the same time because of proximity to other traffic. This is why airports like San Francisco have such bad delays in foggy weather, the runways are only about 800 feet apart. So, they can only use one at a time. New airports like Dallas and Denver have huge spacing between all their runways, so that bad weather doesn't really affect the number of runways in use.
As for greatest number, Dallas/FTWorth and Chicago O'hare both have 7. Large airports like LA and Atlanta have 4 parallel runways, the two outside ones are used to land, and the inside ones are used for takeoff, overcoming the "too-close" problem. Old airports like PHL, BOS, and EWR have long, but close runways and are bad news on foggy days.
So, there are some other factors for airport design.
-Matt, FAA certified aircraft dispatcher.