In the Mexico City airport, the government engaged in an extensive restructuring program that includes the addition of a new second terminal, which began operations in 2007, and the enlargement of four other airports (at the nearby cities of Toluca, Querétaro, Puebla and Cuernavaca) that, along with Mexico City's airport, comprise the Grupo Aeroportuario del Valle de México, distributing traffic to different regions in Mexico. The city of Pachuca will also provide additional expansion to central Mexico's airport network. Mexico City's airport is the main hub for 11 of the 21 national airline companies.
How many national airlines are there in Mexico?
21