20, 5... Number of prac exams don't really matter,
The whole point of practise exams is to find weaknesses so that you can improve.
Are you happy with the results that you are getting with the VCAA prac exams? If yes, probably wouldn't make a major difference.
Since there isn't a lot of time left, make sure you go through and make sure you understand how to do every single question in VCAA exams from 06 onwards.
Anything you aren't sure of . , or topics you feel you aren't 100 percent confident. Get some more practise of those questions.
If you still have time, try Hefernan and MAV papers, since they are quite similar to VCAA. Don't worry about the number of exams. Because at the end of the day, the 2014 vcaa exam will be different, and have different questions to any prac exam out there.
Not trying to scare you, but it is very likely that there's going to be a few questions that most people would have not seen despite how many prac exams there are.
So prac exam is not about trying to expose yourself to every single question out there. It's about learning the skills, knowing the concepts, and how to apply it in exam conditions.
Also: go into exam 2 with an action plan. (Exactly what you're going to start with, and how much time you are willing to spend on each section) Because prepare for an exam 2 that might be challenging, where you need to use your time really well. (I felt like I was pressed for time)