"Trial by 13 good men & true..." and the notes to Ratchett

13 people stabbed Ratchett, not 12.

Before someone who misses the point says, "The Andrenyis stabbed him together," - duh, yes. So 12 stab wounds - by 13 people.

Also, at the end, Poirot, to drive the importance of 12 home, asks (and different passengers answer), "How many wounds in Ratchett's body? 12. How many people in a jury? 12. How many capital letters, each inscribed by a different hand, were in each of the threatening letters? 12." But each letter plainly wasn't written by a different hand, and again, there were 13 inscribers, not 12.