I'm only in Year 10 but this question has been pondering in my head for a while. I asked a teacher and she said that it was best to chhoose 12 because you have a greater control of how well you do in your HSC. Remember that only 10 units count towards your HSC so if you screw one up and your doing 10 units, then that's bad. If you do 12 on the other hand, you can still keep improving the other ones. Choosing 12 units can also keep you motivated to do work. Some subjects may also relate to each other in a way e.g. Biology and GEography, so you can draw some knowledge in from other sources. I'm only in Year 10, I dont know if this is true but some people have done 10 units and got really high UAIS. if you give it your best shot in all these subjects, you wont go wrong.
Good luck