If you are new to IntelliJ Android Studio and wondering how you can enable line numbers for files, well you can do that by simply doing Right Click at the Gutter of any file and selecting Show Line Numbers.
Note : This will not enable line number for all files!! If you try any other file you would see that the line numbers are missing.
Enabling Line Numbers for all Files
1. Go to Android Studio : Preferences.
2. Go to Editor tab.
3. Expand Editor's tab and select Appearance.
4. Under Appearance check Show Line numbers.
Follow the video :