That's more than 14 atoms. Either that or Captain 'merica's shield is so little vibranium it makes no difference.
I don't know what to say about that first part. Yes, it is. Your research is
Brownian in accuracy.
Obviously Cap's shield has a significant amount of vibranium in it.
Francium has 14 naturally occurring atoms on the planet.
I wonder who counted them all?
The Xantalos Department of Making Up Numbers?
As to how they actually figured it out, Francium is naturally formed only by radioactive decay. Figure out how much of the uranium ores containing francium there is and how fast they turn into Francium, then how long it takes them to turn into whatever they turn into.