The number of unpaired electron(s) in $M{g}^{2+}$ ion is:

The electronic configurations are given below:

${}_{12}Mg:\begin{array}{ccc}K& L& M\\ 2& 8& 2\end{array}$
${}_{12}M{g}^{2+}:\begin{array}{cc}K& L\\ 2& 8\end{array}$
(noble gas configuration, no unpaired $e^{-}$)

Hence, there are zero unpaired electrons in $M{g}^{2+}$.