The command line flags -N or --LINE-NUMBERS causes a line number to be displayed at the beginning of each line in the display.
You can also toggle line numbers without quitting less by typing -N<return>. It it possible to toggle any of less's command line options in this way.
You could filter the file through cat -n before piping to less:
cat -n file.txt | less
Or, if your version of less supports it, the -N option:
less -N file.txt
You can set an enviroment variable to always have these options apply to all less'd file:
export LESS='-RS#3NM~g'
You can also press = while less is open to just display (at the bottom of the screen) information about the current screen, including line numbers, with format:
myfile.txt lines 20530-20585/1816468 byte 1098945/116097872 1% (press RETURN)
So here for example, the screen was currently showing lines 20530-20585, and the files has a total of 1816468 lines.
If you hit = and expect to see line numbers, but only see byte counts, then line numbers are turned off. Hit -n to turn them on, and make sure $LESS doesn't include 'n'.
Turning off line numbers by default (for example, setting LESS=n) speeds up searches in very large files. It is handy if you frequently search through big files, but don't usually care which line you're on.
I typically run with LESS=RSXin (escape codes enabled, long lines chopped, don't clear the screen on exit, ignore case on all lower case searches, and no line number counting by default) and only use -n or -S from inside less as needed.
Source: Stackoverflow.com