  Recent studies on superconductivity in NbSe$_2$ have demonstrated a large
anisotropy in the superconducting critical field when the material is reduced
to a single monolayer. Motivated by this recent discovery, we use density
functional theory (DFT) calculations to quantitatively address the
superconducting properties of bulk and monolayer NbSe$_2$. We demonstrate that
NbSe$_2$ is close to a ferromagnetic instability, and analyze our results in
the context of experimental measurements of the spin susceptibility in
NbSe$_2$. We show how this magnetic instability, which is pronounced in a
single monolayer, can enable sizeable singlet-triplet mixing of the
superconducting order parameter, contrary to contemporary considerations of the
pairing symmetry in monolayer NbSe$_2$, and discuss approaches as to how this
degree of mixing can be addressed quantitatively within our DFT framework. Our
calculations also enable a quantitative description of the large anisotropy of
the superconducting critical field, using DFT calculations of monolayer
NbSe$_2$ in the normal state
