Hey!
I'll address the questions you're asking :-)
Should I spend an equal amount of effort on all 12 units or just the 10 units and a slight effort on the 12 units?
For sure! You don't know what could happen - who knows, maths could count towards your ATAR. It's worth it at the end anyway, knowing you got through all your subjects and maintained that effort all the way through.
Does my HSC which has the bands of the subjects impact me in anyway in the future? Do any universities use that or is it just the ATAR that universities use?
Unis tend to use ATARs, but there are pathways which don't really need your ATAR (early entry looks at marks). Bands in HSC subjects are usually used for bonus points, so for example: Band 4 = 1 bonus point, Band 5 = 2 bonus points, Band 6 = 3 bonus points. These bonus points help you out a bit for entry, as they add on to your ATAR. A lot of the time people get into courses using their ATAR and bonus points, so that's that.
Is there any circumstance in which either international universities or Australian universities would use your HSC?
Um... I honestly don't know. 😂It's probably just your ATAR.
Hope this helps!