The number displayed in an enumerate environment is controlled with the enumi counter. If the enumerate is nested in another one, the counter used is rather enumii, and so on.
For example,
\documentclass{article}
\begin{document}
\begin{enumerate}
\setcounter{enumi}{8}
\item First item
\item Second item
\stepcounter{enumi}
\item Third item
\setcounter{enumi}{17}
\item Fourth item
\begin{enumerate}
\item First subitem
\setcounter{enumii}{5}
\item Second subitem
\item Third subitem
\end{enumerate}
\end{enumerate}
\end{document}

Also, keep in mind that you can always specify manually the label of the item between square brackets. For example, the last output is the same if the lines
\setcounter{enumi}{17}
\item Fourth item
are replaced with
\item[18] Fourth item