Let $\text{Inj}(X \to X)$ be the
D2222: Set of injections from $X$ to itself. Since $X$ is finite, result
R1855: Conditions for endomorphism on finite set to qualify as permutation shows that $\text{Per}(X) = \text{Inj}(X \to X)$. Therefore, applying
R1854: Cardinality of the set of injections between finite sets yields
\begin{equation}
|\text{Per}(X)| = |\text{Inj}(X \to X)| = \frac{|X|!}{(|X| - |X|)!} = |X|!
\end{equation}
$\square$