>>27998678>>27998777>>27998835I wouldn't say there's "millions" of Mewtwos, Ho-ohs, Groudons, Xerneas, etc..
But I wouldn't say there's only one of them.
Legendary pokemon are most likely as powerful and as rare as they are territorial and secluded and the more powerful they are they rarer and less there are if only because the environment can't support hundreds let alone even DOZENS of these sorts of creatures hanging around in one location.
Legendary Pokemon like Mews, Shaymins, The Elemental Birds (Moltres, Articuno, Zapdos) and maybe even the Regis & "Elemental Beasts" I wouldn't doubt at all if they exist in numbers ranging in the hundreds of even thousands, but they're just spread so thin and across such vast territorial distances.
Then you've got Mythical Pokemon like the creation trio, Xerneas, Groudon or Deoxys and as powerful as they are there's most likely only going to be a handful of them or maybe even only 3 or 5 at any given time in the world.
This is how I'm rationalizing it at any rate.