IIRC, from the Tea Party rally last year, the NPS doesn’t normally provide figures. They might give some (as you state) ballpark figure based on non-statistical observation, but that’s all. I doubt there’s any way one can closely estimate the number attending.
But the photos speak for themselves.
As Glenn said on his Friday morning radio show, they count the number of buses and extrapolate from there. It ain’t what a CPA would do, but it’s close enough for our purposes.
The other day on Glenn Becks TV show he held up a picture of the Mall. He said they have calculated that the Mall can hold 200,000 thousand people and that grassy area on the left next to it can hold another 100,000. We can also see there are people in areas to the far right.