Pokémon Storage has officially been increased in Pokémon GO. Niantic is now allowing players of its immensely-popular mobile game to hold a maximum of 3,500 Pokémon. To gain more storage, you can purchase more Storage Upgrades via the in-game shop. This number should be more than enough for all types of players!
Attention, Trainers! The limit on Pokémon storage has now been expanded to 3500.
Source: @PokemonGoApp
Pokémon Blog 
Not true, as of December 2020, it’s still 3000 items
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