Maybe this is getting off topic, but I think just like the number 16 has special significance on Scadrial (and for the cosmere at large), the number 10 keeps coming up on Roshar. 10 silver kingdoms, 10 heralds, surges, orders of the KR, 10 evil mega-spren, the list goes on. It might be a chicken and egg type scenario, but it's definitely significant (do we know if there are only 10 groups of highspren for the 10 orders?). So maybe the 10 essences and the other 10s reflect something about the creation of Roshar or it's place in the cosmere.
As for foil, maybe they understood "metal" as a whole to be made of layers of foil when they saw how it was formed while mining?