The oxidation number of H in ${\text{MgH}}_{\text{2}}$  is

In${\text{MgH}}_{\text{2}}$ , Mg is a metal and H is a non-metal elements.

In all metal hydride compounds, hydrogen shows negative oxidation state (-ve) where as metals show positive oxidation state.

Since Mg belongs to IIA group it shows $+2$ oxidation sate only.

Oxidation number of H in ${\text{MgH}}_{\text{2}}$ ,

$\text{1}\left(+2\right)+2\left(x\right)=0$

$2x=-2$

Therefore, $x=-1$ .