\documentclass{article}
\usepackage{xepersian}

\makeatletter
\def\mycount#1{\expandafter\@mycount\csname c@#1\endcsname}
\def\@mycount#1{%
  \ifcase#1\or اولا\or دوما\or سوما\or چهارما\or پنجما\or ششما\or هفتما\or هشتما\else\@ctrerr\fi}
\makeatother

\renewcommand\thesection{\mycount{section}}

\begin{document}

\section{}
\section{}
\section{}
\section{}
\section{}
\section{}
\section{}

\end{document}