i would say the magic number is 11, because you at least have 1 stuff up unit. even if the one not counted is 1 of a 2 unit course, it isnt that much more work (depends on the subject, but like 3u english isnt that much more, or german and other languages etc) and could be worth it. 12 is ok, and more than that, i think you could better use your time on your other subjects and improve them and not need such a big 'safety net'. my school wouldn't allow us to take more than 12