Historically, HK protestors grossly over exaggerate for optics, but MSM usually endorses organizer headcount uncritically. Reality consistently closer to police (under)estimates. Professionals at crowd control are better at estimating crowd sizes. Study from HKU of pre 2019 protests:
https://graphics.reuters.com/HONGKONG-EXTRADITION-PROTESTS/0...
Another analysis with machine learning, about 1/2 protestor estimates.
https://www.nytimes.com/interactive/2019/07/03/world/asia/ho...
And another, estimates protesters had to occupy 18 miles of streets to pack 1M protestors, actual protests occupied 1 mile. Acknowledges not exact science, but leans towards conservative estimate of 250k... incidentally close to police estimates.
https://www.cjr.org/analysis/how-many-marched-protests-hong-...
IMO just average protestor and police reports for ball park figure.