The ID number is the original trainer's ID, the person who caught or hatched the Pokémon.
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The ID number of any Pokemon is given to it by the trainer - namely, you. Your ID number is given to every Pokemon you own. So, no specific Pokemon has any specific ID number. It is the trainer that gives the Pokemon the unique ID number.
Pokemon have the same ID number as their origanal trainer.
That is your trainer's ID number.
Mewtwo's Pokemon ID number is #150 in the national Pokedex.
That's your Trainer ID number, not a Pokedex number.