From the Washington Times (similar stats on other media outlets):
For the 21st year in a row, the two-wheeled crusade called Rolling Thunder has taken over the capital of the free world. An estimated 350,000 motorcyclists — plus their intrepid passengers, activists, organizers, fans and awestruck spectators — have assembled here to draw America's attention to fallen soldiers, lost warriors, prisoners of war, honored veterans and military families.
It's true enough--someone came up with the "estimate". The only difference between Rolling Thunder and the "pro-life" or "anti-war" or whatever rallies and marches is the existence of an opposition with a motive to question the accuracy of the estimates. Rolling Thunder seems to be a sacred cow.