I just estimated - I know 16 of us got offers and I think between 20 and 25 people applied, so I went for 2/3, but the real figure might be a lot closer to 3/4. Can't help but feel a fraction or percentage without context is meaningless, though - for example, I'd argue my school is far better at getting people into Oxbridge than a school at which only the one or two "cleverest" pupils were allowed to apply, and both got in. At my school, anyone who was keen was encouraged to apply, and many of the candidates, myself included, weren't anything special on paper.
Not that I mean to imply criticism of anyone else's school, or to imply that all schools with 1 or 2 applicants just prevent other people from applying (having been at a bog-standard comp from 11-16 I realise this definitely isn't the case!).