I would call it 475-525k easily based on the photos I see there. A lot of people under those trees and depending on what time the shot was taken..if it was taken at 10 am then it isn’t anywhere near what it turned out to be. Some people I know weren’t able to get there til 11:30 because of the congestion and people were still showing up.
I can go along with that estimate. It’s difficult, if not impossible, to be accurate in totals given the ebb and flow of the crowd.
As to the time of the photos, check the shadows—particularly of the Washington Monument.