count-words is an interactive compiled Lisp function in `simple.el'.

(count-words START END)

Count words between START and END.
If called interactively, START and END are normally the start and
end of the buffer; but if the region is active, START and END are
the start and end of the region. Print a message reporting the
number of lines, words, and chars.

If called from Lisp, return the number of words between START and
END, without printing any message.