Unless it's XII. Or, more accurately, XXII.
The first Call of Duty game was released on PC in October 2003. The next one will be out in November 2012. In those nine years, we've seen the series make the jump from the PC to the - deep breath - Mac, PS2, Xbox, GameCube, PSP, DS, Wii, PS3, Xbox 360 and iPhone. There have even been Call of Duty games released on the N-Gage and BlackBerry.
Across all those years and all those platforms, there have been nine "core" games in the series. Four set in the Second World War, three Modern Warfare titles and two Black Ops games. When someone says "Call of Duty", that's normally what they mean.
But there have been plenty more. The first Call of Duty game on console, 2004's Finest Hour, was a spin-off from the PC original, not a re-release. Call of Duty 2: Big Red One did much the same for the second "core" game, taking the general setting of the PC sequel and turning it into a mostly-new game on console.
Later console adaptations, at least during the PS2 era, did much the same thing, as have the handheld ports of titles like Modern Warfare. Hell, the Blackberry version of Black Ops may be called Black Ops, but it looks like an Amiga game.
There's even been time for an expansion pack, United Offensive, which added more singleplayer missions to the original PC game's campaign.
All those games and all those platforms mean ten different studios have worked on the franchise, some like Infinity Ward and Treyarch as principal developers, others like Raven and Spark Unlimited on porting duties or helping out on things like multiplayer.
So how many games in total does that leave us with? It's hard too say! There's a muddy line between many of the spin-offs as to whether they count as all-new titles or as elaborate ports, but by my reckoning - and this is taking each individual release that's not a direct port as a standalone game - there have been nineteen Call of Duty games released.
That's counting everything, from the PC to the consoles to the N-Gage to the Blackberry to the two iOS zombies titles. The only title I've left out is United Offensive, seeing as it was an expansion pack.
Which, if you want to get particular, would mean Black Ops II could really be called Call of Duty 20.