You can usually start off with four subjects for AS and then decide at the end of the year if you want to drop one in year 13. I did biology, chemistry, maths and psychology in year 12 but, even though I did well in psychology, I decided I would have struggled to continue with all 4 so I dropped psychology. It all depends on how much you need the subject you're considering only doing to AS. If you decide to drop one subject after year 12, see if you can do something like an EPQ in year 13 which will give you an extra qualification but will take up less time. I don't think it would affect your chances with oxbridge too much because I only did 3 A levels and an AS and I got an interview at oxford, and there were other people being interviewed who were doing like 6 A levels. I think they care more that the subjects you do are relevant and you are doing well in them.