Hey Want2fly,
You will need at least 6 months on your CPR, so it's best you apply to do it very soon, the people checking the dates of issue at Virgin will be satisfied if you are booked to attend and have a receipt to show them that St John mail you within a week, just to prove you are keeping updated.
It seems to be roughly 30 or so people at the recruitment days and in both my experiences [been for DJ twice] it was roughly half of the people left at the end of the day, I think you'll find it seems to be the common amount.
There's actually a really informative post I was looking at and contributing to along with a stack of other people last year that is still very in tune with the recruitment process [bar maybe a different role play scenario etc] which you can find here:
Virgin Blue Recruitment (Merged)
You can't post on it as it's a locked thread but you will find a wealth of information throughout it! Hope that helps. Any more questions, post away!
Good Luck!