% A complete graph
% Author: Quintin Jean-Noël
% <http://moais.imag.fr/membres/jean-noel.quintin/>
\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage[nomessages]{fp}% http://ctan.org/pkg/fp
\usepackage{tikz}
\usetikzlibrary[topaths]


\begin{document}
    \newcommand\n{5}
    \begin{tikzpicture}
        %the multiplication with floats is not possible. Thus I split the loop in two.
        \foreach \number in {1,...,\n}{
            \node[vertex] (N-\number) at ({\number*(360/\n)}:5.4cm) {};
        }

        \foreach \number in {1,...,\n}{
            \foreach \y in {1,...,\n}{
                \draw (N-\number) -- (N-\y);
            }
        }
    \end{tikzpicture}
\end{document}