How many legendary Adventurers are out there in-story?

Pretty much your own PC is the unique snowflake, even the other adventurers you take to Castrum Meridian, Pretorium, and Crystal Tower are technically considered your 'lackeys'.

Spoiler below from a CS i notice in pretorium

If you watch the exit scene with a party that also has it (and must be on your server), you'll notice maggie comes for you, and also your 'lackeys' will still hang around. Since there's only one magiteck your character makes the ultimate decision to leave the lackey's behind and take Thancred with you on the escape.

Whether they silently decided to lay down their life for your escape or if it's simply just lazy scripting, is up to you. I personally see it as the former.

Similar to what others have said, I see it as yes, their are other adventurers like you. Some have the Echo, some even manage to get jobs and do amazing feats. In your story though, you took on all the primals and the empire, maybe not alone (Ifrit being the only real exception unless other Echo adventurers had been captured before hand), but you probably always delt the killing blow while leading the charge. By Prae, its probably you, maybe 1-3 adventurers with the echo, and the rest being the best of the best adventurers.

Also on Maggie, I personally see it as you riding normally with Thancred where he is in the cutscene, while the rest are hanging on for dear life on various parts (with at least one having the time of their life.)