Considering there will be a third hall open this year, we believe around 12,000 tickets have been made available (this number has also been reportedly confirmed by phone sales reps), which is 50% more than the 8,000 that were sold last year. The World Wide Invitational that took place in June also sold out very quickly.
In the case of the WWI, additional tickets were made available after the primary batch had sold out. However, and this is important, there is no indication Blizzard will do this again.
We'll update with any further information when we have it.