It was not a static crowd. People appeared to be arriving and leaving throughout the duration of the event. From my perspective there were significant numbers along the length of the Mall. Also, two nearby Metro stations service the Capitol area. All of which complicate crowd assessment. I agree, I would lower bound the number at around 500,000 but 2 million does seem a bit high (this somewhat based upon my recollection of aerial images of past events).
Is it possible to come up with a percentage that the people we see in the video represent compared to the entire event?
I.e. what percentage of people who attended did the actual march versus arrived at the event a different path. If we guess that it was 60%, then the current minimum of 250k extrapolates to a total attendance of 250K / .6 = 416K.