An important thing to consider/remember with respect to completing past exam papers is to ensure you are able to identify, understand and work on improving yourself in areas in which you made mistakes. This should be prioritised because completing as many past papers as possible without effectively learning from your mistakes and applying what you've learned to the next paper is not an effective way of preparing for both your trial and HSC exams. However, if you feel you are consistently able to thoroughly determine and work on areas of improvement each time you complete a past paper, then I think you should attempt as many papers as possible, particularly for maths because as mentioned above, each paper may have different and unique questions. In this case, completing as many past exam papers as possible allows you to be exposed to as many types of questions and approaches as possible, which constitutes good preparation for your trial exams.
I hope this helps!
