At the sixth form college my daughter has applied for, 30 people took Class Civ in 2021.
Compare that with 193 for Maths, or 177 for Psychology, it seems a fairly small number. However if we look at Spanish (20) or French (10) it definitely seems more popular than modern foreign languages.
I think it probably doesn't get picked as often as say History, because people know what History is - they've studied it in school. GCSE Class Civ is much less widely offered in state schools, so a lot of the time students stick with subjects they already took beforehand.
From a personal anecdotal perspective, A Level Class Civ is a fascinating subject. The smaller class sizes can only be a good thing if you ask me, as you'll have more opportunity to have questions answered by the tutor, and to get assistance if you need it.