Why Illustrious did not fill every aircraft space with Swordfish

If Illustrious could carry more than thirty aircraft, why did she not use thirty-six Swordfish at Taranto?

Because she also needed Fulmars and Sea Gladiators to defend the only carrier and its task force; a second carrier would have enabled a larger strike.

Illustrious carried twenty-four Swordfish, fifteen Fulmars, and a small number of Sea Gladiators while using deck parking. Three Swordfish were unavailable after contaminated oil damaged their engines. With only one carrier present, fighter protection for the task force was critical, which left no room to exchange those aircraft for more torpedo bombers. A two-carrier force could have pooled fighter cover and released more deck capacity for the strike. The hull's maximum aircraft capacity was different from the balanced air group required by the actual operational risk.