So report some news outlets (e.g., CBS New York), quoting a New York Post story, which likewise makes a similar claim. But the key is in a sentence a bit lower down: “Not all the murder cases have been solved, though, so the number could go higher.”
Could it ever! According to the New York Post story that the CBS story cites, “Police solved 152 homicides in 2013,” out of a total of 334. That means 182 homicides weren’t accounted for, and the total of stranger homicides could thus be anywhere from 29 to 211 (29+182).
I suspect that stranger homicides are more common among the 182 unsolved homicides than they are among the 152 solved ones — in a non-stranger homicide, the killers tend to be easier to identify, precisely because they come from a pool of the victim’s family members, friends, and acquaintances (though note that, as the New York Post mentions, “[a]n acquaintance can include a rival gang member”). The total number of stranger homicides in New York City is thus likely to be a good deal higher to 29, and perhaps closer to 211.
So it seems to me that saying that “only 29 victims did not know their killer” is quite misleading, even if later on one mentions the possibility that “the number could go higher.” The actual number — as opposed to the number police have identified so far — likely already is a good deal higher than 29, and focusing on the 29 will give readers the wrong impression.
(Note that the New York Post report uses “homicide” and “slaying” interchangeably with “murder,” though the two are different — homicide and, presumably, slaying could include heat-of-passion manslaughter (or its New York state law equivalent), self-defense killings, and possibly even some accidents. The New York Post doesn’t report on the definitions that it used, though I suspect that all these terms are used to refer to murder plus heat-of-passion manslaughter, since that seems to be the definition used in New York City homicide reports. That’s another source of possible uncertainty in the numbers, though not the one I’m focusing on in this post.)