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The only truly unique and one-of-a-kind Pokemon is the Original One, responsible for creating the entire Pokemon multiverse. Due to the infinite nature of the multiverse, there are infinite amounts of all Pokemon. And as we are players who can interact with indefinite instances of these universes and timelines, we can achieve those limitless numbers to our benefit.
As it also turns out The Original One is also one-of-a-kind because it's uncatchable, as being able to catch it would require there to be multiple Original Ones, thus implicating a greater Pokemon Omniverse of which an even greater entity was responsible for birthing the multiverses and their Original Ones.