1. 1. Go to Tools on the menu on the top of SSMS.
   2. Click on Options.
   3. On the box that opens, on the left menu, select Text Editor and then Plain Text.
   4. Click on Line Numbers.
   5. Click on OK and and you are ready to go.

More information can be found at https://docs.microsoft.com/en-us/visualstudio/ide/reference/how-to-display-line-numbers-in-the-editor.