How is true "1 of#" determined?

Is there any criteria for determining the actual production number relevance when describing a vehicle as a 1 of #? I mean, there could be a car that has 2500 equally optioned counterparts,,but one happens to have a power antenna for example. It could be a 74 barracuda, 318 at bench seat, but it is a power antenna optioned car,,,does that make it a 1 of 1,,because it is the only one of 15000 74 barracuda 318 at bench seat with a power antenna? (I have no idea about these figures, it is just for conversations sake). What options actually discern a vehicle from other similarly optioned vehicles when it comes to production number claims of rarity?