How many hairpins are there on the 13.8 km climb up Alpe d'Huez, a legendary part of the Tour de France?

Explanation

Alpe d'Huez is a legendary part of Tour de France. The climb is 13.8 km long at an average of 8.1%. It is particularly well known for its characteristic 21 hairpins. Alpe d'Huez has been part of the Tour route almost every year since 1976. The first time it was included was already in 1952, however, when the Italian Fausto Coppi won. Another winner was Giuseppe Guerini who, alone at the front and riding towards a victory in 1999, collided with a spectator. The mountain is sometimes known as the Dutch mountain because of all the Dutch fans.