Top score can show you roughly how many people in your schools previous cohort got a band 6 in a subject (here is the link - http://topscores.info/select-nsw.php).
It will depend purely on the school - for example, top 10 schools can probably have top 40%. At a school ranked 300, you'd probably be ranked 1-3 to have a chance.
This is quite a ridiculous question, with no definitive answer. There isn't a set rank needed for a Band 6 or Band 5 etc.
You can come last in your cohort and get a Band 6, given that all your cohort's HSC exam marks are 90+. You can come first in your cohort and not get a Band 6 given that the highest HSC exam mark is less than 90.