I’m going to US I got USA visa already. But I need to stop in Sabiha and change to Istanbul airport. Do I need transit visa? What is known traveler number?
Do I need transit visa?
Depends on your citizenship. Since you have to change airport you need proper documentation to enter Turkey
What is known traveler number?
The US has programs for frequent travelers such as Global Entry or Nexus. These programs make immigration in the US a lot easier. You don't need one, though. If you are not participating in any of these programs, you can just leave the field blank