I have 12 GCSEs, which was very much the norm in my school, HOWEVER I got 7A*s, which gives me a low proportion of A* compared to how many GCSEs I did.
As far as I can tell, getting a high proportion of A* is more important than a high number of GCSEs, so if you can get straight A*s you're application will be much stronger than getting a bunch of Bs and Cs aswell.