By default line numbers are not enabled in the source tab when working custom control or xpages.

To show line numbers do the following:

1. Go to the Source tab for any XPage or custom control.
2. Right-click on the gray margin to the left of the code. See the red arrow in the screen capture.
   
3. Select Show Line Numbers in the menu.

Once the line numbers are enabled they stay on until you turn them off again.