Based on previous years, after the first stage it was whittled down to the 150 mark. Considering this year's stage 1 was a lot more comprehensive than simply essay writing, I doubt if it is more than circa 150, as they would actually have gotten a better image of what each candidate's chances of suiting the course are compared to essays, it would seem a waste of resources sitting through 2000 90-120 second videos when you already have a rough enough idea of each candidate's abilities.
Essentially, considering previous years went from 3,000 to 150 applicants based on 4 essays alone, it wouldn't make logical sense that a far more detailed examination would actually leave them with 500 after the first stage. Unless somebody knows something the rest don't.