Just curious, how can you tell from your own personal experience whether it's 10m or 20m though? It feels more crowded?
I'm of the view that Shenzhen has around 20m people as well.
1) Many official estimates put the effective population at 20m.
2) Number of official workers in Shenzhen are comparable to Shanghai.
3) It's unlikely that SZ's GDP per capita is 50% more than Beijing/Shanghai when average wages are lower.
https://defence.pk/pdf/threads/chin...ed-in-2018-10-087.590562/page-5#post-11011643
https://defence.pk/pdf/threads/chin...ed-in-2018-10-087.590562/page-5#post-11012882
https://defence.pk/pdf/threads/chin...ed-in-2018-10-087.590562/page-6#post-11013590
A feeling during the commute, I was never squeezed like that in any ~10M city even during worst rush hours. Same on roads. And that even if you make a discount for Shenzhen being a rather densely built up even by Chinese standards.
And I will trust professional demographers after all. They did a survey few year ago during Ma Xinrui's administration.