Sorry, Ik know i'm slightly late to the party. I technically did 5 alevels being Maths, Physics, Computer Science, DT: Product Design, AS Further Maths, EPQ. The reason is, I was doing 4 A-Levels and wanted to go to Imperial/Oxbridge for maths and computer science, both who required Further Maths or self study + the STEP or MAT tests, and **Imperial wanted 4A*. My alevels were a lot of coursework which took a bit of pressure off the exams and by no means do you need to do 5. If your university requires that you do a specific alevel, then offer to do the AS. It shows that you are willing to put the effort in and they will understand that 5 alevels is alot of workload and they won't ask you to do that (make sure you mention your situation in your personal statement). Alternatively, if you are in y11 and just starting alevels, look at the uni course you want to do and the alevels they require then do those 3 + one that you enjoy. Also when doing 4, there is no need to do an EPQ as, by doing 4, you have already shown that you can time manage. If you really want to do 5, even if it's just for ego, do some subjects which include coursework so you can remove pressure from the exams, and do subjects you enjoy. Just a final point, if you are having to study before every topic test and only getting As or having to put extra effort in outside of class then an extra A-Level isn't for you. If you are easily getting As or 90-100% on all subjects just off classwork then have a look if you must.
**On UCAS these unis will request A*AA. Bear in mind this is the MINIMUM grade and is what applicants are expected to get if they have a bad day in the exams. Talking to many of the unis the admissions test will filter out all the students below A*A*A* and most of the time they will go with the 4-5 A Level students as they have so many applicants they can turn straight A* students away. You really need both a good personal statement AND top grades. (Just to put this into perspective, the head of computer science at Imperial said not to even think about applying to Maths and Computer Science Joint honors without 4 A-levels and most students have top grades) I don't think this is for every oxbridge course but if you're looking for maths/science/cs course then I imagine it will.