I am doing 5 AS levels, though one of those is further maths - for people who really enjoy maths and find it quite easy, I imagine it's similar to doing a language you're fluent with regards to the work you'll have to put in. Personally, I wouldn't say doing 5 AS levels has been a problem (I also do History, English Lit and Economics), though I will probably drop English Lit for A2. I would say take 5 if you think you can get AAAAA. If you're hoping for AABBB, then I wouldn't recommend it, just because I expect you could do better concentrating on 4, and unis prefer quality to quantity (within reason). I suppose the only exception would be if you felt sure that, even if you only did 3, you still would only get AAB. And I wouldn't worry too much about A2 now - it's better to leave your options open with regards to dropping subjects, based on how you've done after AS.