I have a MacBook Air (13", Mid 2011). What macOS is the latest one I can use?

Click on this link to read how to get different versions of macOS --> How to download macOS - Apple Support

- The article also has links to system requirements for each OS version which you should check against your exact computer model.

This web site also has a reliable presentation of model and OS version compatibility: https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

MacTracker says that the latest version of macOS that will run on a MacBook Air (13-inch, Mid 2011) is High Sierra (10.13.6).