Mew is number 151 in the original National Pokédex.
Mew is a Mythical Pokémon introduced in the first generation and classified as a Psychic-type species. Its National Pokédex number places it immediately after Mewtwo, which is number 150. That ordering made Mew the final entry in the original 151-Pokémon index.
Mew was not normally obtainable through ordinary gameplay in the original Pokémon Red, Green, and Blue games. It was distributed through special events and promotions, which helped create a strong sense of mystery around the species. The hidden Pokémon became one of the most famous examples of an event-exclusive character in the early franchise.
Mew is sometimes confused with Mewtwo because their names and Pokédex positions are closely linked. Mewtwo is an artificial Pokémon created from genetic material associated with Mew, while Mew is the naturally existing Mythical Pokémon. The number 151 refers to Mew, not Mewtwo.