We broke the Europa League record attendance with our game against Gent this season. 80,456 out of the 90k available turned up. Given the fact that we were trailing from the 1st leg and it was a cold Thursday night against a mid-table Belgian side, that is pretty impressive. I think with Wembley you are likely to get a lot more tourists who might take the opportunity to take in a game of football at the national stadium - something that was previously impossible save for half a dozen times a year. The waiting list for our season tickets currently stands at around 35,000 people so even the extra 18,000 being allocated next year will only make a dent in that figure. Time of course will tell but if we can somehow figure out how to play decent football in that damned place, we'll fill it without issue.
It's substantially longer than that, IIRC. Something silly, like 60k+. It'll be interesting to see how many actually take up the offer for next season.