Anything really is fine, whether that be 1, 4, 8, 15 or even higher than that. Don't worry about the number, it really is a quality over quantity thing. I would argue that VCAA past exams are all essential to do, however. Use practice exams to learn as much as possible from them (sit most of them in exam conditions, look carefully at all the mistakes you made, go back and read up on topics you struggled with etc.), rather than trying to smash out as many as you possibly can. If you learn what you can thoroughly from the exams that you complete, then there really isn't a need to be doing /all/ the practice exams.
I think around now I was just looking at exam questions here and there and started doing practice exams around August/September (only a couple a week at first, then really got into that exam revision period once September holidays came around). Something like that anyway.