Here is the image of the option window with the Line Number Checked correctly
SQL Server 2017 Express Edition Windows 10 64 bits Following the below path Tools --> Options --> Text Editor --> Plain Text --> Line Number I tried multiple times shutting down the system, not sure what I'm missing here.
Assuming you are working with .sql files you will want to adjust the setting in this path Tools → Options → Text Editor → Transact-SQL → Line Numbers.
See my blog post about this here for more info.