To see the list of routes you’re currently connected to, open up the airport window and click on the “ROUTES” tab at the bottom.
When you connect two cities, you’ll only have one airplane moving back and forth on that route. But, if you want to add more planes to a route, it’s really easy! Just click on the “+” button in any blank cell next to the route you want to add a plane to. You can add up to a maximum of 5 airplanes on each route.