\documentclass{article}

% numbers are stored in counters
\newcounter{tmpnum}

% macro to increment a string (given as argument)
\newcommand{\stringinc}[1]{%
\setcounter{tmpnum}{#1}%     setcounter effectively converts the string to a number
\stepcounter{tmpnum}%        increment the counter; alternatively: \addtocounter{tmpnum}{1}
\arabic{tmpnum}%             convert counter value to arabic (i.e. decimal) number string
}

%example usage
\begin{document}
The number 12345 is followed by \stringinc{12345}.
\end{document}