Do not do 5 A-levels. It probably won't make a difference re Oxbridge admissions - if anything it could actually make it harder because you will be so busy you won't have time to focus on your application, or, you focus on your application at the expense of your studies and risk missing your offer. For Oxford it is not a case of more A-levels are better. Most offers will be based on 3 A-levels. Doing 4 is worthwhile in your case to have further maths in there, but anything on top of that is just unnecessary work and stress. Instead of a 5th A-level consider doing an EPQ if your school offers it and you can manage your time to get it done efficiently, having an EPQ in a topic relevant to the degree you're applying for can give you a great starting point for supercurricular reading for a personal statement (though the EPQ grade itself doesn't count for much at Oxbridge). Or if you don't want to do that/school doesn't offer it, you can still do supercurriculars. It can be anything related to the subject you want to study; read books and articles, listen to podcasts, attend lectures and talks, come up with a personal project to work on - anything that demonstrates your subject interest. Doing that, and preparing thoroughly for your admissions test if there is one, will be way more beneficial to your Oxbridge application than a fifth A-level. Another thing is you don't want to burn out, 5 A levels is a ridiculous amount of work and seeing that through to the end will be a challenge. Its much better to have the free time to focus on your application and also having a life outside of study than getting another A level that doesn't make much difference to a uni application!