I'm not sure if we can say how many of one kind of Legendary Pokemon are out in the Pokemon world, other than the ones who are confirmed to be one of a kind like Mewtwo. There could be a lot out there, but they could live in harsh climates or hide themselves well enough to where people aren't able to find them. That's why most Legendary Pokemon would be considered extremely rare. Though, that's just a guess on my part.
While there is a story behind Lucario due to movie 8, I don't really see how that could help it become a Legendary. For starters, that story is mostly known around that Kingdom not only for Lucario, but also because of Sir Aaron. The other stories revolving around Legendary Pokemon are known throughout their specific region and/or the whole Pokemon world. When we have encountered Lucario in the series after the movie, they haven't connected those Lucario to the one in the movie. When they have seen Legendary Pokemon in the past, the Pokedex entry or what the characters say about it usually relate back to that story.
As for the special ability part, there are a ton of Pokemon that have unique abilities that aren't legendary. The Unown created Molly's dream world in the third movie and allowed Ash, Misty and Brock enter into Larvitar's mind back in Johto, but Unown aren't considered Legendary the last time I checked.
Even if Lucario's nurabers get really low somehow in the Pokemon World, I don't think that it would make it a Legendary Pokemon. It would just make it even more of a rare Pokemon. The fact that it still is an evolved Pokemon really prevents that from happening. Plus, since it was never considered a Legendary Pokemon all throughout the fourth generation, I seriously doubt that they would somehow make it out to be Legendary in the future of the series.
The idea of Lucario's nurabers getting really small also sounRAB pretty sad to me. Not only because I love Lucario, but also because that would mean that fewer Riolu are surviving or evolving into Lucario.
While it is possible that they could be throwing a new curve ball at us with somehow making a legendary Pokemon have a pre-evolution, I still seriously doubt it will happen. That would just go against what they have established for the past four generations. Unless they specifically say that Zoroark is a Legendary Pokemon, I'm just seeing it, along with Zorua, as a special kind of fifth generation Pokemon.
Yeah, it probably was a one time thing since Ditto has only transformed into other Pokemon after its first appearance. It was weird, but I thought that it would be fun to point it out anyway.