Not read the whole thread. I'm getting pretty experienced in this lark as I have 1 at uni, 1 just doing AS and 1 in year 9 just chosen GCSes
All at super selective grammars.
Schools are very sure and practised in the management of very bright kids and how they can succeed best as rounded people, not only academically. Watching DD1 and mates through the system it seems to have worked well.
THey do 10 GCSEs (and the girls have a half GCSE in IT taken in year 9. the boys school rate IT so little as a GCSE that they don't bother, AS computing is taken by both schools in years 10-11 if you want a computer type qualification).
University applications (esp for medicine etc) do not look fot 15 GCSEs they look for the right subjects at A/A*, and actually often would prefer them all taken in 1 sitting in year 11 too.
DS has mates that are new to the school for 6th form that have 15A* or equivalent. When you look into that it's not a university impressing portfolio as the subjects are non mainstream or BTEC type equivalent grades.
The other thing limiting the number of GCSEs does is allow time to have " a life" yep a social life is vital but other educational breadth is there too- musical activities at high standard, D of E, volunteering etc . depending on where he's headed these are all material for personal statements on uni applications and what makes you different to the other candidates with brilliant grades.
Agree you never need 11 GCSEs. The worst situation would be that the 11th drags your overall profile down, either because you are spread too thinly, or you just don't much care for it. 10A/A looks fab. 10 A/A and a C less so.