A note on isomorphisms between submonoids of $\mathbb N^k$ and numerical semigroups

Given a submonoid $H$ of $\mathbb N^k$, we give some characterizations of the minimum $r\in \mathbb N^+$ such that $H$ is isomorphic to a submonoid of $\mathbb N^r$. In the context of submonoids of $\mathbb N$, we prove that if two numerical semigroups are isomorphic submonoids of $\mathbb N$, then they are equal and the identity map is the unique isomorphism between them.