Passage: Turkish Airlines (Turkish: Türk Hava Yolları) is the national flag carrier airline of Turkey. As of 2019, it operates scheduled services to 304 destinations in Europe, Asia, Africa, and the Americas, making it the largest carrier in the world by number of passenger destinations (excluding regional brands). It serves more destinations non-stop from a single airport than any other airline in the world. Turkish Airlines flies to 122 countries, more than any other airline. With an operational fleet of 20 cargo aircraft, the airline's cargo division serves 82 destinations.The airline's corporate headquarters are at the Turkish Airlines General Management Building on the grounds of Istanbul Atatürk Airport in Yeşilköy, Bakırköy, Istanbul. Istanbul Airport in Arnavutkoy is the airline's main base, and there are secondary hubs at Istanbul Sabiha Gökçen International Airport and Ankara Esenboğa International Airport. Turkish Airlines has been a member of the Star Alliance network since 1 April 2008.

Question: What is the main base for the airline that flies to the most number of countries?
Istanbul Airport