Niantic announced on their Pokémon GO Twitter feed that Pokémon storage for Pokémon GO has been expanded to 2000.

Pokémon Storage Expanded to 2000

Attention, Trainers! The limit on Pokémon storage has now been expanded to 2000. Thank you for your patience and good luck on all of your Pokémon GO adventures.

Source: https://twitter.com/PokemonGoApp/status/1065368391257677824