\documentclass{article}

\title{Foobar}

\usepackage{countwords}
\newcounter{words}

\begin{document}

\maketitle

% Start counting after title is printed, so it is excluded from total.
\countwordsstart{words}

There are seven words in this sentence.

% Stop counting before the maths...
\countwordsstop{words}

\[
  e^{i\pi} - 1 = 0
\]

% ... and then start again.
\countwordsstart{words}

And six more in this sentence.

\countwordsstop{words}

There are \arabic{words} words in this document.
% Will print:
% There are 13 words in this document.

\end{document}
