Lots of people say that it's different for everyone and to do as many as you can, but I have no idea how many I should actually do. I plan to just take the average of what people say haha.
Look, these people aren't wrong! I wouldn't say taking the average is a bad approach, but it disregards the fact that it needs to work for you - if you get a skewed result in either direction, you could end up stressing yourself out with overwork or end up going in undercooked. You know your limits - if you've been successful previously, imo you should try and replicate the amount of practice that led you to success, otherwise, try and do as many as you're capable of doing (if you're feeling overworked or stressed at any point, just stop) - Trials after all are just a test run for the actual HSC - so you know even better what to do and what not to do.
Personally I procrastinated either with web browsing, gaming or maths papers, so I stacked procrastination on top of actual practice for maths which led me to do way more papers than I originally anticipated. For my other subjects, new syllabus for everything meant that I did zero past papers and relied on lunch classes/teachers feeding me questions weekly. I could also steal a bunch of resources off friends who did tutoring (more me copying their questions/them asking for help) - which helped me practice as well. 20+ papers is perhaps a little OTT for some people's style of study but if it works out well for whoever employs that strat, it's not my place nor anyone else's to criticise as we all have our own ways of succeeding! As such, no definitive amount - minimum 1 (as a bare minimum, you really should do more though) so you know what you're up against. With the small amount of time, you could try and squeeze in a few more, but really try and focus on exam style questions targeting your areas of weakness