During cellular respiration, one molecule of glucose (C₆H₁₂O₆) requires 6 molecules of oxygen (O₂) to fully oxidize into carbon dioxide (CO₂) and water (H₂O). Therefore, for two glucose molecules, the calculation would be:
2 glucose molecules × 6 O₂ per glucose = 12 O₂ molecules.
So the answer is 12.