don't fall in the trap of doing more units for HSC, what you have to realise is that only 10 of these units you are doing will be counted towards your uai. SO if you are performing well in your subjects (and i don't mean getting over 50%) then stick with 10 units and spend your time concentrating on those units.
On the other hand, if you don't think you are doing well, then stick with 12 units, 13 max. Trust me,I know, my friend did 14 units for her hsc, she was stuffed. 1 unit subjects are crap, they may scale you up, but don't fall into the trap. In order to be "scaled up" you need to get 49/50 for your assessments. If you can achieve this score, than go for it.
I strongly suggest doing 10 units. Less worries, more concentration on subjects u need!