In the chemical reaction, ${\mathrm{Cl}}_2+{\mathrm{H}}_2 \mathrm{S}⟶2\mathrm{HCl}+\mathrm{S}$, the oxidation number of $\mathrm{S}$ changes from

Hydrogen have $+1$ oxidation number. So in ${\mathrm{H}}_2 \mathrm{S}$, the oxidation state of sulphur is

             $$\begin{gathered} 2\left(1\right)+\mathrm{X}=0 \\ \mathrm{X}=-2 \end{gathered}$$

The product elemental sulphur oxidation state is $0$ . The change in oxidation state of sulphur is $-2$ to $0$