Why does a four-aircraft CAP need eighteen fighters?

Why does sustaining four aircraft on combat air patrol require a squadron of eighteen?

Sixteen aircraft make four divisions of four for the operating cycle, while two additional airframes cover losses or heavy maintenance.

The answer turns a headline air-group number into a readiness calculation. Four fighters may be airborne while other four-aircraft divisions prepare, recover, or undergo routine work. A sixteen-aircraft structure supplies four complete divisions for that cycle. Two spare airframes bring the squadron to eighteen and preserve the plan when an aircraft is lost or enters heavy maintenance. The exchange shows why the number on station is much smaller than the force required to keep it there.