It is not an exaggeration. I was there, joining several protests including the CHRF 2M protest. In addition, the police estimates is always wrong (of course they want the numbers to be as low as possible). One example was a protest around Victoria Park. According to “official police figures” it was 180k attending - but they only counted the people in the limited Victoria Park square, it was completely PACKED. They didn’t count the hundreds of thousands of people outside the square who couldn’t fit in, they also didn’t count the people stuck in the MTR next to the park, who couldn’t even fit on the streets. The “official police statistics” is always off by a magnitude of x.

What is your estimate, and how did you measure it?

I live in Paris where most of French demonstrations take place (including the yellow vest movement) but I can't pretend to have better estimates than all local and global media sources and public observers.

So what source do we trust on the numbers?

Satellite imagery is likely the best way to estimate.

The massive crowds in slow march peaceful protest videos I saw were astonishing.

However if we extrapolate that not everyone who'd support it or are silently supporting it - what % of people available to protest/march were out?

Edit to add: downvoting a question asking for trustworthy, quantifiable sources - you're failing.