CTRL – ML toggles line numbers in the CodeMirror area. See the QuickHelp for other keyboard shortcuts.
In more details CTRL – M (or ESC) bring you to command mode, then pressing the L keys should toggle the visibility of current cell line numbers. In more recent notebook versions Shift-L should toggle for all cells.
If you can’t remember the shortcut, bring up the command palette Ctrl-Shift+P (Cmd+Shift+P on Mac), and search for “line numbers”), it should allow to toggle and show you the shortcut.