\documentclass{article}
\usepackage{mdframed}
\usepackage{lipsum}
\newcounter{tempcount}
\NewDocumentCommand \boxpar { O{3} m O{\linewidth} }{%
\begin{mdframed}[align=center,innertopmargin=.4cm,userdefinedwidth=#3]
\setcounter{tempcount}{0}
\loop\ifnum\value{tempcount}<#1
\the\value{tempcount}\rule{\linewidth}{.8pt}\\[2pt]
\stepcounter{tempcount}
\repeat
\ifx\relax#2\relax\else#2\fi
\end{mdframed}%
}
\begin{document}
\lipsum[1]
\boxpar{}
\lipsum[2]
\boxpar[10]{}[8cm]