I went to the information evening, and I'm pretty sure that they said there's no set quota; rather, you just have to meet the prerequisites. The prerequisites are stated on the UniMelb website, but I think this is what you have to do...
Year 11 subjects related to the UMEP study: A-A+ average
Other year 11 subjects: B+ average
Year 12 subjects studied in year 11: 42+ (although they DID say that you could get in with a 38).
In addition, you often have to study certain subjects at year 12 level when in year 11. If you don't, you generally have to sit a test, although I do know that if you're applying for physics and you haven't studied both methods and physics 3/4 you can get the test waived if you've scored well in the physics olympiad
. I'm not too sure about the other subjects though...