The number of unpaired electrons in the low-spin octahedral complex $[\mathrm{Mn}(\mathrm{CN}){}_6]{}^{3-}$ is:

Mn is $3d^{5}4s^2$, so $\mathrm{Mn}{}^{3+}$ is $3d^4$. The strong-field $\mathrm{CN}{}^{-}$ gives the low-spin arrangement $t_{2g}^4{e}_g^0$, which has two paired electrons in one $t_{2g}$ orbital and two single electrons in the others. The number of unpaired electrons is 2.