Original: Strictly speaking it has two runways, however, the northern runway (08L/26R) can only be used when the main runway (08R/26L) is out of use, for example because of maintenance or an accident.
Simplified: It is also often quoted as the world's busiest single runway airport, although strictly speaking it now has a second 'stand-by' runway, which can only be used when the main runway is out of use.