How is true "1 of#" determined?

Galen states "known to exist". At least the few sheets I've seen at car shows. But yeah, it's like saying it's 1 of 1 in my neighborhood. I had 2 1 of 1 cars this summer (taking @pishta way of thinking). My friend has a 71 440+6 4 speed GTX, 1 of 72, according to the bill board he carries with the car, he even put it out at a very informal car gathering. All my cars are 1 of 1, they all have my touch involved, but never the same way!