Call

.........And oxygen is in deficiency.

Given $10$ $mol$ of dioxygen gas, at most, $6.67$ $mol$ of sulfur trioxide can be generated. This is clearly indicated by the stoichometric equation. Should the reaction proceed to completion (which is unlikely) there will be approx. $0.33$ $mol$ sulfur unreacted.

Industrial production of sulfur trioxide (the precursor to sulfuric acid) relies on the Contact Process: oxidation to $SO_2$, and then catalyzed oxidation of $SO_2$ to $SO_3$. It must be an incredibly dirty and smelly process, yet it is undoubtedly vital to our industrial society.