I personally did 16 units in the HSC and it was the worst decision I've made in my life - because I could've used better spent my time, in subjects that I struggled with. Balancing 16 units is quite difficult, especially with some subjects being completed in Year 11.
My advice is, I would drop from 13 units. You have to take into consideration that you will need time outside the classroom to revise the content and to apply your understanding to practice questions, revision etc. Especially if there are harder pieces of content and you do 13 units, you will not be motivated to do well since it will be mentally draining. If you do not know what subject you want to drop, consider the following:
Work with your strengths: In a way, try to be strategic. Drop the subjects that is your weakest. In the long term, this will help you invest other time to subjects that need it most. If you continue to keep the subject that you are weakest at, you will need to spend more time on that subject revising, understanding content, and will be more frustrated with. Initially, I was going to drop Physics instead of English Extension 1, but in reality, I had a better grade and rank in Physics than extension. In the end, I decided to keep both
....Remember, the HSC is a game, and if you want to succeed, play with your strengths and eliminate your weaknesses.
Think about your range of subjects: This may seem like an odd piece of advice, but really think about your subjects. It is important to have a diverse range of subjects because you are able to change the way you study and the skills needed. For example, your sciences would require you to do calculations and understanding the relationships presented, thus, doing a lot of textbook questions or classwork would be much more beneficial if you want to get enhance those analytical skills. However, too much of this can become tedious, and draining. So, you turn to music, where you can become much more 'creative' with how you study, and focus on your major work. In a way, it would act as a good break from the "drilling" from your sciences and maths. This ultimately makes studying much more tolerable as you aren't consistently using your brain for 1-2 main skills for 4-5 hours a night depending on how much you study.
Consider the content itself: A majority of the time leading up to HSC will be you knowing, understanding and applying the content into every day practical situations. So, you must be committed to learning the content and must be engaged with it. So drop the subject that you think you will not be motivated to learn the content, a subject where you procrastinate to be organised with. If you hate the content, then you will not enjoy it. There is no point in being in a class where you can't put time and dedication, where you know yourself it could be better spent.
I hope this helps! Feel free to message if you need more advice!
