How are runways numbered?
Runways are usually numbered according to their direction, more precisely called runway magnetic bearing or QFU (see Q codes).

Consider a plane flying toward the runway on final approach in a day without any wind. Divide its magnetic heading by 10, round it to the nearest whole number and you'll usually get the runway number. For example, if the magnetic heading is 345° then 345/10=34,5, so the runway number will be 35 which will be a runway used for landings (and takeoffs) to the north.

Opposite ends of the same runway have different numbers, 18 (which represents 180 degrees) apart. A runway with 35  for landings to the north will have runway 17 for landings to the south. Even though these are the same strip of concrete, they are treated as separate runways by pilots and controllers.

If there are two airports near one another with runways at the same angle, sometimes one of the airports will add or subtract one from the runway number to help planes differentiate between the airports.

Occasionally a runway number will change when the magnetic declination angle changes in such amount, making the runway magnetic bearing divided by 10 and rounded to the nearest whole number increase or decrease.

Some runways in areas of large magnetic declination use true instead of magnetic headings for the runway numbers. This is not unusual in northern Canada and Greenland.

When there is more than one parallel runway at an airport, L, R, or C may be appended to the runway number for Left, Right, or Center. These are based on the approach direction, so, for example, the runway 35L would be called 17R from the opposite direction.