how to turn on number lines in nano code example

Example 1: nano with line numbers

nano --linenumbers [filename]

Example 2: display number of lines in nano linux

Press 
Alt+# 
or
Alt+Shift+3

Example 3: nano add line numbers

according to the man page for nanorc, you can add set linenumbers in ~/.nanorc, /etc/nanorc, $XDG_CONFIG_HOME/nano/nanorc or ~/.config/nano/nanorc.