Hey there,
I think it depends on what subjects you do. I started with 14 (including both english and maths extensions, physics, chem and bio, and music) but I felt like the workload was way too much (with a job/hobbies etc). So I dropped bio (because I found it the least interesting), and stayed with 12 units. I thought 12 was maybe a little too much as well - but that's considering 3u maths and physics (which didn't start out being my forte). So naturally, in year 12, I dropped chemistry (I found physics the most interesting science), and picked up Ext 2 English (English was my favourite) so I ended up with 11 units. Honestly, I think 10 would have been perfect for year 12, but I figured my back up unit may have been useful.
It just depends how you feel the workload is for you during year 11. Think of it this way, you can always drop a subject, but it's much harder to pick something up. So see how you go and push yourself, but not to the point where your studying/learning becomes counter productive. If you genuinely enjoy a subject, you'll probably be more inclined to want to study for it, so stick with subjects you are both good at, and enjoy.
Good luck with everything!