Unfortunately, attendance to something like this won't be anywhere near 100% of the people who express interest. Especially considering the prestige of MIT and the publicity, I bet a lot of people are considering it but ultimately won't be able to go because it's really far and a long event around midterms time.

For reference (orders of magnitude different, I know), I organized a local hackathon and heard from probably 100 'interested' people. We ended up netting 30 registrations, which was the cap for our venue, but I only actually turned 2 people away. So 2/3 of the developers that I spoke to and emailed personally couldn't make it, and that was in the city they already live in.