I heard there were significantly more than 600...
As one of the 250 of so who were lucky and got out of GBS during the lost round at Xmas, I heard a rumour that the number of volunteers was somewhat closer to 1,000 than 600.
Even so, at 600 thats 1/5.5 of the target population. If it was 1,000 applicants then that would make it 1/3.3 which is a tad embarrassing.
Anyway those days are long behind me (well around 11 to be exact).