How many legendary Adventurers are out there in-story?

[Melithea]

That said, I also subscribe to the idea that the player-character just happens to be the lucky one among a handful of adventurers chosen by Hydaelyn after the Calamity. I recall, for example, that in my character's first vision of the the Mothercrystal, he wasn't alone — there were other adventurers in that vision, who were also presumably blessed with the Echo. It just so happens that our characters were the ones who caught the attention of their respective Scion representative, and was thus recruited into the organisation.

This really does highlight the plot hole in the lore vs. mechanics. If there were several other people with the Echo running around who were also elite adventurers chosen by Hydaelyn, they would all find their way to the Scions eventually, or be getting hunted by the Ascians, or both, yet they are never mentioned. And apparently none of them make their way into your Praetorium showdown group? It's just a different group of elite adventurers? So what's the point of Hydaelyn choosing many champions? Is it like buying 20 lottery tickets to improve your chances that you get a winner?

I see our character as being in an elite class of talented individuals, akin to Olympic athletes. Only a small percentage of people are born with this level of potential, and only some of them ever meet it. You, the gold medalist, are getting that extra little 'oomph' from Hydaelyn and the Echo that pushes you into epic status. Other PC and some NPC adventurers make up the remaining Olympic-athlete tier adventurers like the ones who previously made up the Warriors of Light (other than yourself). But there's still a problem when you talk about 'how many'. There are thousands of PCs running around on a given server. Realistically it looks like we outnumber native Eorzeans. Like if we just got organized we could take over Eorzea for ourselves in a matter of days. Yet somehow nobody notices this dramatic fact, and this population is hardly considered significant enough to even merit any effort to mobilize them against the Empire, where the beleaguered Grand Companies are forced to take up most of the work and only because YOU take the time to go out and take care of some business they couldn't.

All this considered, I think the population of adventurers is supposed to be pretty small, completely ignoring the actual number of PCs. Remember that there is a distinct group of NPC adventurers as well. Eorzean lore seems to be set up to largely ignore the existence of most players except for you. At most, a small handful of your fellow players appear to even exist for game purposes, and these also appear to make up a large percentage of the most skilled and successful adventurers around.