Mapping from Set to Ordinal Class is Bounded Above

Theorem
Let $x$ be a set.

Let $\On$ be the class of all ordinals.

Let $f: x \to \On$ be a mapping.

Then $f$ has an upper bound.

Proof
Let $I$ be the image of $f$.

By Union of Subset of Ordinals is Ordinal: Corollary, $\bigcup I$ is an ordinal.

But by Union is Smallest Superset, each element of $I$ is a subset of $\bigcup I$.

Thus $\bigcup I$ is an upper bound of $f$.