I think the amount of GCSEs you takes should be proportional to your acedemic ability.
I say this because I sat my GCSEs at a low-performing secondary school, which achieves an average 5 A*-C pass rate of 40%. There, every student takes a minimum of 12 GCSEs, with some taking 13.
At my sister's school, a private school which achieves an average 5 A*-C pass rate of 100%, they take 9 GCSEs.
Shouldn't the school with the bright pupils be taking more, and the school with the low-achieving pupils be taking less? Someone who is predicted straight Es cannot be expected to sit 20+ exams in 12 subjects.