\documentclass{article}
\usepackage{luacode}
\usepackage{amsmath}
%------------------------
\begin{luacode}
local x = 0
function add()
    x = x +1
    tex.print(x)
end

function sub()
    x = x - 1
    tex.print(x)
end

function reset()
    x = 0
end

\end{luacode}
\newcommand\add[0]{ \directlua{add()}}%
\newcommand\sub[0]{ \directlua{sub()}}%
\newcommand\reset[0]{ \directlua{reset()}}%
%-------------------
\begin{document}
\reset
\add
\add
\sub
\add
\end{document}