8. A country has 21 cities, and some airlines can implement air transportation between these cities. Each airline connects pairs of cities with non-stop flights (and several airlines can operate flights between the same two cities at the same time). Every two cities are connected by at least one non-stop flight. How many airlines are needed at least to meet the above requirements?
To make this country form an aviation network that meets the conditions required by the problem, there must be at least 21 airlines, because the total number of non-stop routes is no less than $20+19+\cdots+3+2+1=210$, and each airline provides $4+3+2+1=10$ non-stop routes. Figure 5 is an example of a service route map for a country with 21 airlines (here, the cities are the vertices of a regular 21-sided polygon, and the non-stop routes are its sides and diagonals). The first airline provides services to cities 1, 3, 8, 9, and 12, while the other airlines provide services to cities obtained by rotating the above cities by $k \cdot \frac{360^{\circ}}{21}$ $(k=1,2, \cdots, 20)$. The satisfaction of the problem's conditions is derived as follows: the segments connecting the vertices numbered 1, 3, 8, 9, and 12, including the sides of the 21-sided polygon and all possible lengths of diagonals, appear exactly once.