One of the greatest gatherings in the world is the Maha Khumbh Mela, at the 2013 gathering there were 30 miliion people. This particular gathering occurs every 12 years, and is popular among devout Hindu worshippers who come to bathe in the holy waters of the river at the sacred site of the Kumbh Mela. It is considered the largest gatheing of humans on earth. The tent city is so large it has been photographed from space. It would be amazing, and clearly important to those engaged with the devotional aspect, but perhaps overwhelming too.
What stands out for me is that place and story are so significant for many people and provides religious and cultural connection at a very deep level. You don’t get 30 million on a whim.