Quoted By:
Ash's Pikachu's full number of appearances in the Pokemon anime:
Main series: 1242 episodes + 20 movies + 1 special + 27 shorts
WPBS: 3 episodes
I Choose You AU: 3 movies + 1 episode
A Ripple in Time: 1 short
Planetarium specials: 7 specials
Hoopa's Surprise Ring Adventures: 6 shorts
Grand total: *1308 appearances in the Pokemon anime
*Counter includes flashback and fantasy cameos (main anime and the WPBS episodes), the movies, the Pikachu shorts, The Mastermind of Mirage Pokemon, the planetarium specials, the Hoopa shorts, The Mewtwo Strikes back CGI remake, the I Choose You AU and A Ripple in Time.
Episodes Ash's Pikachu didn't appear in: 2 Journeys episodes, all 4 mega evolution episodes, 3 WPBS episodes, The Legend of X, Y, and Z!, Hikari - Setting Off on a New Journey!, and Dent and Takeshi! Gyarados's Imperial Rage!!. Not sure if mentions count, desu.
Not everyone Ash/Main Characters/Companions/Rivals/Antagonists/Supporing characters/CotD have ever been shipped with (human/Pokémon + M/M and M/F as a whole) can even beat that.
