Is an archmage somehow who has mastered all six primal sources?
I made a Theory post a long time ago where I went into depth about what the prefix “arch” means.
There are two definitions for that word, one meaning clever, trickster, clever etc., and another that means chief, head, king etc.
Now the first definition of Arch would fit to Aaravos, who’s referred to as the Archmage, but it wouldn’t make sense for the archdragons.
The second one is the more obvious choice and one that applies to the Archmage and the Archdragons: head, chief, principal, best and greatest at what they do or what they are.
But evident from me using archdragons plural, having the prefix arch- doesn’t automatically mean there’s only one.
As we’ve seen, there’s a sun archdragon, an ocean archdragon and an earth archdragon. And there were even multiple sky archdragons, Zubeia and Avizandum, and Zym is said to one day grow into another sky archdragon himself. So Archdragon is more of a category of dragons rather than a title that one individual dragon can hold.
I’d imagine that it would be similar for mages as well. Besides Aaravos (whom most of the world officially doesn’t even know exists), no one has truly wielded multiple primal arcanums before, since every magical being is supposedly only connected to one source. And since all the primals offer a different set of abilities, it would be silly to try and determine which ONE mage is better than EVERY OTHER MAGE OF EVERY OTHER PRIMAL. So up until Aaravos appeared (and until he makes his official return to Xadia) there would be an Archmage for each primal source, making a total of 6 Archmages amongst the elves.
So usually an Archmage would be someone who’s the best at their designated primal. But Aaravos throws all of these principles and norms out the window, as he tends to do with rules.