How many miles is it from the North Pole to the South Pole?

The distance between the North Pole and the South Pole "as the crow flies" is around 20,014 kilometres, or 12,416 miles.

If it was possible to travel directly through the earth, it would be 12,800km or 7900 miles.

N.B. The exact point of intersection of the Earth's axis and the Earth's surface at any given moment, is called the "instantaneous pole," but because of the "wobble" this cannot be used as a definition of a fixed North Pole (or South Pole) where distance and precision is required.

Incidentally, when the revolutionary French were reworking their system of weights and measures on a scientific or rational basis, the distance from the North pole to the equator on the meridian through Paris was adopted as the basis for the measure of length called the meter, in that they selected ten million meters, (or ten thousand kilometers which means the same thing) for this distance. Later surveys showed this length to be slightly in error, but it is an easy way to remember the Earth's circumference, for example: 4 times 10000 km. = 40 thousand km.