I think that's waaaay too many. I thought I read somewhere that DCA capacity is around 35,000.... Definitely less than 50,000.
I think it'll be crowded but less than what it'd be opening day. Not helpful I know but I don't think well really know for sure until we're there.
EDIT: You're right about the official rules though. It states that a MAX of 3,500 invites will be awarded per time slot, but that it depends on the winner's party size. To me, that makes it sound as though there will be 3,500 people in each time preview. As in, if each person only had a party of 1, then 3,500 invites would be sent... but that if everyone had a party size of 2, then 1,750 invites would be granted.
Those numbers just seem more reasonable to me. Here's the actual text from the rules:
"Maximum of Three Thousand Five Hundred (3,500) GRAND PRIZES for each Select Time (depends on each winner's Party Size [1 to 6 Members]). Maximum of Ten Thousand Five Hundred (10,500) GRAND PRIZES for all three Select Times."