I'm actually doing five A2s (English Lit, French, German, History, Latin, as well as General Studies which doesn't really count), having got GCSEs of 3 A*, 6 A, 1 B, 2 C, which isn't really all that great.
While it is a lot of work, and you do feel rather dismissive of all these people who complain about their workload with three subjects, if you feel up to it it's definitely worthwhile, and will probably help when you apply to uni - tutors etc. will know that you can handle that amount of work and can manage your time effectively and everything, but they will probably still only ask for three grades in an offer (so even if you're doing five subjects, they'll only ask for, say, AAB - at least, that's what they did for me), which gives you more flexibility - so my relatively low history marks won't matter, as long as I do ok in three of my other subjects.
Basically, if you want to try doing five and it fits into the timetable ok, go for it. Everyone finds the first term tough, so stick at it and see how it goes. If it really gets too much, pack one of them in. You'll have gained a term's experience of that subject and will suddenly fail like you now have a huge amount of free time.