Without more details, we can give only general advice.
Dropping subjects for Year 12 gives its advantages and disadvantages. Advantages include more time for your other subjects, less subjects to concentrate on in the HSC exam block and if you were doing badly in one subject, then it removes the burden of that subject.
Disadvantages include you being more dependant on the rest of your subjects, and if you only have 10 units, then you have to do well in all your subjects to get a good UAI, there is no back up if you stuff up an exam.
It all depends on your circumstances whether or not you should drop a subject, so good luck in whatever you decide to do.