The roof top was a guess on my part .. but they did have an over head shot of the crowd that reached down the parkway
As for the exact number .. I'm not an expert so I'll leave that to others to decide
If the big shot is accurate, and the shot that was taken from the stage and used in another post is accurate, it looks about the size of a pre-Clinton rally in Gainesville, Florida, where I and others freeped in 1992 (way back when there was no freeping, just free Republicans), including a draft-dodging duck. There were about 30K people there listening to BJ and the Bore, all told.
New rule of thumb with liberal events: Take the 'fire department' estimate. Divide in half. Multiply by 3/5ths.