In the self-reduction step in lead extraction from partly roasted galena, the number of moles of $\mathrm{PbO}$ that react with one mole of $\mathrm{PbS}$ is

The reaction is $\mathrm{PbS}+2\mathrm{PbO}\to 3\mathrm{Pb}+\mathrm{SO}{}_2$. Sulphur ($-2$ to $+4$) loses 6 electrons and each lead(II) gains 2, so 3 lead(II) are needed in total: 1 in $\mathrm{PbS}$ and 2 in $\mathrm{PbO}$. Hence 2 mol $\mathrm{PbO}$ per mol $\mathrm{PbS}$.