Activision Blizzard, in its entirety, has an estimated ~9,642 employees, based upon payroll processing figures. Source:
https://www.macroaxis.com/invest/ratio/ATVI--Number_of_Employees
In 2012, Blizzard had laid off ~600 employees from its 4,700 employee base according to a separate earlier Gamasutra count, bringing it down to ~4,100:
https://venturebeat.com/2012/02/29/blizzard-entertainment-lays-off-600-employees-as-world-of-warcraft-shrinks/
There have been several reallocations of resources and significant departures since that time, given the continuing decline in Warcraft populations and challenges capturing gamer market share among its eSports initiatives with HOTS and even Overwatch.
Finally, consider Activision Blizzard’s Quarterly Filings from 11/8:
https://investor.activision.com/financial-information/quarterly-results
Blizzard Entertainment accounts for ~30% of the operating margin of the entire parent company. It’s safe to say that it is on the higher side of ~3,000 - ~4,000 FTEs, but this number is likely to decrease as Activision Blizzard is pressuring Blizzard Entertainment to become more profitable. This ordinarily comes through EBITDA, reducing operating overhead (costs, including employees) while continuing to drive revenues. More on the culture shift at Blizzard in this Kotaku expose:
https://kotaku.com/the-past-present-and-future-of-diablo-1830593195
TL;DR – Somewhere between 3,000 - 4,000 employees, most likely, subject to continuing decreases and voluntary and involuntary employee attrition as Blizzard Entertainment strives to show profitability. Still the largest and most profitable U.S. game developer.