Istanbul Airport has 277 passenger destinations in May, including both non-stop and one-stop and international and domestic, obviously mainly because of Turkish Airlines. Other airports come close, notably Frankfurt (275), Amsterdam (260), Paris CDG (248), and Dallas Fort Worth (242).
The geographic position of Istanbul drives high narrowbody use. Turkish Airlines, in particular, serves many thinner destinations that wouldn't otherwise be feasible, especially in Europe, Africa (including Mogadishu), and Central Asia. Flights also tend to be more frequent, helping to drive connectivity, competitiveness, and dominance.
Some 56 airports are served in Western Europe, with London Heathrow, Paris CDG, and Frankfurt having more flights than the others. A further 41 airports are served in Central and Eastern Europe and the Caucasus, led by Baku, Moscow Vnukovo, and Bucharest.
Usually, this subregion would have more destinations, but there are currently no flights to Ukraine for an obvious reason. Six airports in Ukraine were served pre-war, including Mykolaiv, which temporarily replaced Kherson.
Turkey is large geographically and often has slow overland travel despite relatively short distances. The longest route is to Hakkari (YKO on the map below), near Iran and Iraq, some 866 miles (1,394km) away. Istanbul is well suited to a comprehensive domestic network, both for P2P demand and connecting traffic.