I did 15 in my prelim year. This year, I've dropped economics and taken on 4U english. So still doing 14 units.
BEST CHOICE OF MY LIFE HOWEVER!!
Ext 1 and 2 English have less class time together than economics, which gives me a few frees a week. Because I love English so much, it's not very stressful for me.
So, bottom line is?
A few extra units don't hurt. And if they're extension units, and you learn how to balance everything out, it can work out really well.
I like the security of knowing that if something goes drastically wrong in my HSC year and I bum out on a subject or two, I'll have back-up.