How many people are allowed in Gare du Nord at any one time?
I don't know if there's an established number of people allowed in the building at any time.
Roughly 1.7 million trips are taken through Gare du Nord each day. That translates to about 95,000 trips either starting / ending / passing through that station each hour (1600 per minute on an 18 hour operating day). This flow of passengers per minute is estimated as if travel didn't have rush hours in the morning and evening, which are likely 3X busier than the average. If I had to guess, Gare du Nord at peak hours has about 5000 people in it per minute, which is still not shoulder to shoulder capacity, just very busy. My guess is the station could hold twice that amount at any moment so roughly 10,000 people, on the major train platforms/departure areas, RER platforms, Metro platforms, tunnels and other areas.