You are fucking insane! I wanted to do 14 (keep all year 11 subs and pick up MX2) but the school only has a timetable for 13 units max and SOR2 and MX2 are on the same line for half the classes so they wanted me to drop to 1 units but I didn't want to end up dropping physics later anyway and only have one back-up unit so I dropped physics and now I'm coming 3rd in sor2 or something without going to class half the lessons so I'm happy.
I think 12 is enough, 13 is pushing it and 14+ is insane.
EDIT: read the above post. Drop physics it's shit.