If you mean starting NOW– I think rather than aiming to complete full exams at this point in the year, completing a few practice exam questions on the topic you are studying for each subject and getting them marked would a more productive and efficient revision strategy. I personally have borrowed a copy of the Checkpoints book for each of my subjects from my local library and I try to complete a couple here and there– doing a few at a time and working my way up to full exams helps digest the content better and not burn myself out trying to answer questions we have not covered in class yet.
If you mean later during SWOTVAC season– for HHD I didn't do full practice exams until the week before my exam, and in the end I only completed 2 and a half fully-timed exams.
I also know a few friends of mine in Further Maths last year did practice exams every day and found it really helpful. I don't think there is a definitive answer on how many practice exams to do– I guess it really depends on the subject and the person. I'd say starting now, try starting off exam prep with a couple of questions at a time (maybe aim for around 5-10 a week?) and see how you go.