That said, it’s not going to be a straightforward affair, if what’s been shared so far is anything to go by. You can only battle as part of a league, and there are three leagues announced – Great, Ultra and Master leagues.
— Pokémon GO (@PokemonGoApp) November 30, 2018 The first two leagues come with a CP limit for each Pokémon. Great League Pokémon may only have a maximum CP of 1500, while Ultra League contestants can have up to 2500. For those who have Pokémon with more CP than that, they can only be entered into the Master League. It also looks like you can have a team of up to three Pokémon for all three leagues.
— Pokémon GO (@PokemonGoApp) November 30, 2018 Considering the lowest CP limit is at a pretty lofty 1500, it would take awhile before a new player gets to a point where they can realistically experience this element of the game. No other details are present at the time of writing, so hopefully Niantic adds a blog post explaining in further detail its implementation of Pokémon battles. (Source: Niantic / Twitter [1], [2], [3] via The Verge)