Hi - there were around 900 last year - possibly even less. I can't remember exactly as it seems a long time ago. However my son had a candidate number around 1200 and he has a P surname.
I don't know how they issue the candidate numbers but they do not represent the number sitting the test IYSWIM. I remember commenting last year that there were nowhere near that number of boys, even accounting for the two sittings.
Why they do that I have no idea - perhaps to ram home the point that it is such a competitive school or just to add a little more angst to the process?