If you can maintain your ranks with the amount of study you do, then by all means 13 units is fine.
I did 13 units and still got 99.90. Though throughout the year, I had thoughts of dropping Japanese Extension and Physics. I actually even stopping going to Japanese Extension classes for about 3 weeks, because I told my teacher that I was gna drop it, but I never filled out the form. So after the 3 weeks I told my teacher, nahh I think I'll keep it, and it ended up counting. So yeah it really depends on you.
If you enjoy all of your subjects then it doesn't matter if you're doing 13 units.
Personally, I really hated Physics, but I was coming like top 15 (out of a group of about 90) for first assessment and half yearlies with like minimal study, so I was like, okay maybe I'll just keep it because I'm getting good without study. But you
can't have that mindset, ESPECIALLY for trials and HSC, it just does not work like that lol. I then screwed up the 3rd assessment, getting 75% when the average was like 95% lol and then I just gave up for trials and HSC. I just kept it after trials because I said to myself, I've already come this far, might as well finish it, and it wasn't a hindrance to my HSC timetable as well so yeah.
I know quite a few people that were doing 12/13 units and got 99.90+ for 09 HSC. So yeah.
Hope that helps