I was thinking about this. The "seated" capacity of Allstate Arena (Rosemont Horizon for us old farts) is 18.5K.
I'm sure no one is surprised that I did some fact-checking on this topic!
I'm pretty sure that the seating at Maiden's last Allstate Arena show was like this:
*Maybe* they went one more section to the left, but definitely the whole left end of the seats was unavailable.
The listed capacity for basketball is 17,500, so the 18,500 figure must refer to concerts in the round (17,500 in the stands + 1000 on the floor). Thus, due to the end-stage setup, Maiden's last show there had to be well under 18,000.
For FMBA, apparently there are "over 12,000 reserved seats". I counted the seats for sale on the LiveNation interactive seating chart, and came up with ~10,500. Add in the skyboxes, seats that don't appear on the map, and the pit if it was filled with seats, and I could see this number reaching 12,000.
From my position on the lawn, the seats were completely full. Even all the "obstructed view" seats all the way in the back.
So then the question is, were there 6,000 people on the lawn? Initially, I would say no, because the density wasn't all that high. But once I started walking around, I realized how truly huge the lawn area is, and I'm not used to doing crowd estimates in such large spaces.
So I'd go 50/50 on the statement that there were 18,000 there. It wouldn't surprise me if there were, and it wouldn't surprise me if that was fudged a little. But Bruce saying it with such authority tells me it was a ticket-sales number that he was given.
Either way, I'd give 90% confidence that it beat their last show. Then the only remaining question is, have they ever done a multiple-night stand in Chicago? I almost thought that they had for the Powerslave tour, but Wikipedia says no. But maybe they did on another tour? If so, this would technically still be the largest single-day show (it clearly beat their 2003 show at FMBA), but perhaps not an indicator of their peak in popularity.
Neil