Why twelve aircraft is a normal carrier squadron

Is a twelve-aircraft British F-35 squadron unusually small?

No; twelve is a standard squadron size, and carriers create strength by combining multiple squadrons into an air group.

Nine to twelve aircraft was a normal squadron range, with some wartime units reaching eighteen. British experience suggested about eighteen fighters were needed for useful air defense. That total could be organized as one large squadron or several smaller ones. Multiple twelve-aircraft squadrons provide group-level mass while retaining manageable unit structure and room for attrition.