i go to a selective school thats ranked top 20 in the state, and were actually DISCOURAGED from doing more than 10/11 units. 12 is possible, but just look at it this way; everyone argues 12 is good cause you MIGHT choke and mess up in the exam, you still have 10 units of good marks to fall back on
BUT almost everyone i know doing 12 units pretty much found a subject they were totally over in the year, and gave up on it without dropping it- essentially giving them the same amount of work, and time in school without the amazing free periods, for a subject they were PLANNING to do no work for.
dont bother with it. i say 10, which is what i did- but you need BE SURE you like those subjects and your good at them. Why i did maths ill never understand, and i can assure you despite hours and hours of work through the year, it will most definitely be dragging my atar down a fair bit =[
so maybe 11 units is a safe bet, cause then if that one unit subject is a higher mark than the last 2 unit subject, they combine the marks and average it- better for you.