For me I would start at 13 units because if you're not sure which unit to drop leave it for now. If you begin struggling during the year than drop to 11 units. For me I'm picking up an extension course which would make it my 14th unit. Crazy, I know but I am going to drop one of my extension courses. I find 10 or 11 units risky because if you cannot perform very well in a subject it would still have to count towards your ATAR. If you are doing 13 units, it is manageable as long as you are organised, make use of your time like before school even. with that said, the advantages of 11 units is obviously one subject less and study periods. However, 13 units is manageable if you make good use of your time. Like come early to school and do work, that will make up for your lack of study periods. The problem with study periods is that you have a tendency to procrastinate more and become lazy because you either rely on your study periods to do hw or you waste your study periods. But that's just some people. Lol. hope I helped