Breaking a list into multiple columns in Latex

Using the multicol package and embedding your list in a multicols environment does what you want:

\documentclass{article}
\usepackage{multicol}

\begin{document}
\begin{multicols}{2}
\begin{enumerate}
    \item a
    \item b
    \item c
    \item d
    \item e
    \item f
\end{enumerate}
\end{multicols}
\end{document}