Maybe I'm being unfair, but compared to the whole Pokedex—which has over 600 Pokémon, at this point—the number of Pokémon Ash has owned over the years seems laughable.
Here's an image that breaks down what Pokémon Ash has caught and when in the show, along with who his (human) companions were at the time.
Click to embiggen.
Interestingly, Ash seems to favor water-type Pokémon, but has never owned a steel-type or a psychic-type. Weird. And he's only owned one ghost-type? C'mon, Ash, those are the coolest ones!
Apparently the show is turning 15 next year. Boy do I feel old. I still remember when Ash set Butterfree free....