When you are editing in SQL Server Management Studio Line Numbers would be very useful specially if you are working on large module.
To enable line numbers in SQL Query Editor, follow below steps:
Step1: Go to Tools > Options
Step2: In the Options dialog box navigate to Text Editor > Transact-SQL > General
Step 3: Check “Line Numbers” and click on “OK”
Now when you re-open your SQL Editor you will be able to see the Line Numbers. See image below.
That’s it! Hope this helps!