Line numbers are not displayed by default in most Visual Studio installs. To enable, select ‘Options…’ from the ‘Tools’ menu.
Now from the options dialog, expand the ‘Text Editor’ section and click ‘All Languages’. Then simply check the ‘Line numbers’ box in the Display section.
Now you should see line numbers in the text editor when you open a file.
Note that the screenshots from this example are using Visual Studio 2010, but these steps apply to many previous versions as well.