Personally I started off doing 5 A-levels and by the end of two months I gave up on one of them. I think it is a LOT of work doing any five subjects, and although I did humanities I can imagine science based subjects would be even more work.
I think if you really want to do 5 A-levels you have to extremely motivated and dedicated, but also, if you want to get decent grades, extremely smart.
You could always start with five and drop to four, as I did?