I noticed that Chicago Intl has 8 runways. What is the criteria for the number of runways? Many international airports still have one. I can understand the reason for two but what are the factors to create more? If one was to develop a new international airport today, what number of runways would they default to if there's such a default? What are the benefits?
Amount of traffic. Busy eventually equals more runways.
Like your local interstate highways. Some places, 2 lanes. Some places, 8 lanes.
"International" doesn't mean much as far as amount of traffic. My local regional airport, PHF, is "international". It has a flight to Bermuda once a day. There are 2 runways, but one is rarely used.
In the U.S., and as noted in another answer, "international" in the name doesn't (shouldn't?) have anything to do with the size or the amount of traffic. Rather it is used to designate that the airport is an "Airport of Entry." In other words, there will be U.S. Customs and Border Protection there.
Insofar as the number of runways, I suggest that four is a good number to work with for a busy international airport: one for departures and one for arrivals for operations to and from the airport for 0° through 179° and another pair for 180° through 359° or whatever allocation makes sense.
I will try answer point by point:
What is the criteria for the number of runways?
- Amount of traffic is the criteria. The more traffic you have the more runway you need.
Many international airports still have one. I can understand the reason for two but what are the factors to create more?
- They have more traffic than 2 can handle. BKK currently has 2 runways but master plan is for 4 runways to accommodate increasing amount of traffic.
If one was to develop a new international airport today, what number of runways would they default to if there's such a default? What are the benefits?
- There is no default as it depends on amount of traffic. International Airport means it has custom and immigration. If the traffic is light one runway is suffice.
- If you ask how to plan for a large airport I would suggest a plot of very large empty land for future expansion and good connection to cities you serve.
As everyone else has mentioned, amount of traffic is paramount. Airports commonly operate with several parallel runways active -- with departures and arrivals sometimes separated, but often interwoven.
In addition, the variability of winds can lead to a need for more runways. At Ohare (Chicago's international airport), two of the runways are not parallel to the other six. So in many regards, it really only has six runways that can operate simultaneously.
Atlanta has five runways -- all parallel. Denver has six - four North/South, two East/West.