Technically, 2. If you wish to do more than 2 subjects, you need to get approval from the head of division. However, for any business, econ, finance or accg subjects, you are not allowed to do more than 2 subjects. Also, for those commerce subjects, you are not allowed to take 2 subjects where their classes clash. At the end, you are left with very with options with regards to commerce subjects.
EDIT: here for more info:
http://www.coe.mq.edu.au/eligible.htm