Let's review: there are twelve stab wounds on the victim and twelve people on the jury. A coincidence, dear Shmooper? We think not. Take it from Poirot:
"I remembered a remark of Colonel Arbuthnot's about trial by jury. A jury is composed of twelve people – there were twelve passengers – Ratchett was stabbed twelve times." (3.9.58)
The number links the passengers on the train with the idea that they are acting as a jury (and judge, and executioner) bringing a murder to justice instead of simply killing out of revenge. As Poirot tells us:
"Ratchett had escaped justice in America. There was no question as to his guilt. I visualized a self-appointed jury of twelve people who condemned him to death and were forced by exigencies of the case to be their own executioners. And immediately, on that assumption, the whole case fell into beautiful shining order." (3.9.59)
As Poirot suggests, the notion that the passengers are a jury of twelve does offer the simplest solution to the case.