I think it's best with 11 or 12 units.
The way things go, you really can't predict what can happen throughout the year, even if you do get a start on your Majors now.
I was supposedly the favourite with Ext2 and I ended up dropping it.
The subject is challenging and a time vacuum, and requires a LOT of independent work.
Not to say that you won't deliver in those areas, I think it's best to have a couple of just-in-case units. You'll be glad to keep a really easy subject that doesn't require much work to maintain if stuff doesn't work out- such as the Gen Maths. You'll find with a little bit of application, EVERYTHING in the syllabus is learnable. Your ability to keep afloat in a subject like that would reflect a good level of independence when it comes to what's required of you in the Ext2 course.
In the end I did 11 units, having dropped ecos at the end of last year and Ext2 about a term and a half in. In an unexpected turn of events, Ancient History, which I was pretty good at, turned out only getting mid 70s by my Sem2 report, with a shocking rank. It turns out that my trial mark was shite.
So, what I thought to be safe went to hell, and unless I do well in the HSC exam, I can kiss a sweet unit goodbye.
Turns out that even 11 units is a risk, to be honest.
So, to conclude my two cents, I'd recommend doing more than 10 units for two reasons.
One, you may think so but you don't know how well your majors will go, nor your other subjects. Ancient betrayed me somehow.
Two, a change of heart in anything throughout the year can affect your performance. The ability to shave off subjects once you've had a real taste is invaluable, and you may find that a certain subject within your 10 units isn't what you'd hoped it'd be like.
The safety barrier thing is underrated I think, but really without it, I know a lot of people would be far more worried for their UAI, because things can happen.