I think a lot of people showed up but if there was people from Azadi square to Imam Hoessein square, that is 10KM, i.e. 10,000 meter * 30 meter wide (which is much less but let's do that to take care of the squares) that is 300,000 square meter. If there are 2 per square meter, then 600,000 people showed up in my calculations.