Any answer based on "official" statistics would be wildly
inaccurate. Due to the Hukou system of China in particular, and the
local ID Card scheme of Shenzhen city to a lesser degree, you have
official government numbers that do not really match the amount of
people who live and work in Shenzhen.
The last official Shenzhen census in 2010 put the population at
around 7.2 million. Now, include all the people who consider
Shenzhen their home but have their Hukou in another city or
province, and the blue collar "migrant" workers and the real number
is closer to about 12 million.
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Old Answer = The population of Shenzhen on November 28th 2011 is
approximately 8,923,582. (Extrapolated from Hong Kong populations
of 6,940,000 in 2006 and 6,980,412 on October 9th 2010, and a
Shenzhen population of 8,912,300 on December 30th 2010.)
Source: True Knowledge