Prove that if $n \in \Bbb N$, $f: I_n \to B$, and $f$ is onto, then $B$ is finite and $|B|\leq n$.
Notes on notation:
For each natural number $n$, $I_n = \{i \in \mathbb{Z} \mid i \leq n\}$.
$A \sim B$ indicates that $A$ is equinumerous to $B$.
$f: I_n \rightarrow B$ means there is a function from $I_n$ to $B$.