Depends which subjects you take and how much you're willing to work. I take 5 (Maths, Further Maths, Chemistry, Biology, Physics) but due to some overlap in the subjects it isn't as much of a workload as 5 humanities subjects. I have significantly more work than my friends but if you are organised and willing to work then it shouldn't be impossible.
See if your sixth form will allow you to take 5 subjects for a few weeks and then drop one - you might have a better idea of what you want to do after trying each subject out at A level.