The stoichiometry is 2 moles $SO_2$ to 1 mole $O_2$. With 5 moles of $SO_2$ and 1 mole of $O_2$, oxygen is the limiting reagent. 1 mole of $O_2$ will react with 2 moles of $SO_2$ to produce 2 moles of $H_2SO_4$.