Doing 5 in year 12 is fine. I find that frees are useless anyway. But it really depends on how much work you are planning to do and also how your school timetables, doing 5 may leave you with very little spare time at school because the timetable is geared for people doing 4. Additionally, if you are considering an enhancement subject, it is quite a small workload.
I'm doing 6 and it is going fine. It could get hectic if I had done a heap of work for all of them, but at the moment it is fine.