It's 48 credit points per year, so generally it is 24 per semester. Most first year units are 6 credit points, so you would end up taking 4 units per semester, although as maths units are only 3 credit points at this stage, science people tend to have 5 subjects per semester.
As a general rule, the number of credit points taken tends to mirror the number of hours spent at uni for each unit (for science units, at least).
Edit:
http://www.science.usyd.edu.au/about/handbook.html - 'undergraduate enrolment advice and policies'
Page 18 and 19 has the info on the BSc/BA (going on past posts I am guessing that that is where you are headed if you end up at USyd). Other faculty handbooks should have similar sections.