Inside the World's Biggest 'Magic: The Gathering' Tournament
rollingstone.com
rollingstone.com
Utrecht was really well organized. It was the first 'pro' tournament I was attending, and everything went quite smoothly. I strongly advise you to do it if you have a chance (especially during those kind of monstruous events, which may be a bit less pro-oriented than usual).
So the weekend saw about 16,000 players in the various GPs themselves, though of course there were more people total when side events and spectators/traders/etc. are counted.