So how many shared bikes are needed?
Beijing is the city in focus.
In April 2017, Beijing (北京市交通委) published a guideline (draft for consultation) to regulate shared bikes. [A usage survey conducted at that time in Beijing]
Beijing had around 700k shared bikes by April 2017, and more than 1.6 million by September 2017. (numbers are for registered bikes, not including non-registered)
The number of shared bikes reached its largest in September 2017 (2.35 million; then new deployments were suspended). Other cities also set the cap (Shanghai 1.5 million, Guangzhou 0.9 million, Nanjing 0.45 million).
The number declined to 1.91 million by August 2018, which was the second hard limit set by City of Beijing. (mobike 0.699, ofo 0.907, other 7 companies 0.3)
It was expected that the cap would continue to decline. From January to May 2019, only 1.2 million of the 1.91 million bikes are used at least once.
Other cities have downsized the number of shared bikes as well: main area in Chengdu 1.8 million -> 0.75 million; Shenzhen 0.89 -> 0.6 million
Beijing has a population of more than 21 million: so on average every shared bike will cover 11 people. (using 1.91 million here)