I would say you can solve this by going from a supply or a demand side.
I would probably say that I would go from the supply side and try to see # bands capable of hosting 50K+ events. I would say using my own country (~10M) as a referrence point there are ~10 bands capable of filling a venue like that. Using it was a proxy (1 bands per 1M) for the entire world there will be ~8,000 bands capable of doing that.
Usually a band like that goes on tour ~4 months per year, making 1 mega show per week, meaning that each band will have 16 mega events per year. Taking into account the 8,000 bands that we mentioned we will have 128,000 mega concerts in the world per year.
As a sanity check I could say that my countries population is roughly thousandths of the entire world meaning that there are ~128 mega concerts per year there. taking into account the country has less than 10 venues able to holding a crowd that large having roughly ~13 nights fully booked per year seems a bit low. Most likely we underestimated number of mega concerts 1 band does per year.