At one point during the very early stages of the HSC, I had a total of 15 units after picking up both 4U English and history extension - having such an insane amount of units was actually quite benefiting as it allowed me to decide which subjects I enjoyed most and would excel at. Not long afterwards, I dropped 2 subjects which brought me down to 11 units and a lot of free periods which were used to study.
I recommend deciding which subjects you enjoy most and believe you're going to excel in then drop the ones you least favor as the HSC progresses - that's the real benefit of having more units than you require.