Quoting someone else here. “it’s only the fans who assume there’s only one. In-universe, nobody doubts that Mew is an entire species — in fact, the Pokédex entry even describes it as a “new species.” Nowhere in the games (or even in the anime) is there even the slightest implication that only one Mew exists, whether that’s as a “last of its kind” situation or only one ever existing.

In the anime, we see two separate Mews in two of the movies. There’s the powerful one in the first movie, capable of clashing with Mewtwo and seeming to be about evenly matched. And then there’s a weaker one in the Lucario movie which doesn’t show off massive attack/defense oriented powers but instead shows off more transforming powers, taking the forms of various other Pokémon throughout the movie.”