The number comes from "CLS and several other Chinese agencies," as well as from the China Passenger Car Association (CPCA).
Update Feb 9, 2021: The new MIC Model 3 number from CPCA is 13,843, (we guess that the remaining 1,641 is for the MIC Model Y).
* CPCA reports wholesale shipments, not registrations/deliveries.
Anyway, 13,843 is a strong number, above the average for 2020, and above all months besides the two last - November and December.
There is a theory that MIC Model 3 production continues at a high speed, but the sales results are noticeably lower than in December (above 23,000) due to exports.
Troy Teslike reports that around 8,000 of the cars were exported to right-hand-drive markets (mostly UK):
With export or not, the Model 3 is one of the top electric car models in China.