In the Pokémon franchise, trainers traditionally bring six Pokémon with them to battle. Upon catching the seventh Pokémon, it will automatically be sent to the PC for storage.
Players can retrieve boxed Pokémon to PC at the Pokémon Center, but must submit to a member of their team in order to replace it. The exact reason for the six Pokémon rule is not entirely clear.
In the Pokémon Adventures manga, while there is no physical limit to how many Pokémon a trainer can have in hand, only six are allowed to be used in official battles.
According to Professor Oak in his appearance in Pokémon Adventures, this is the ideal max that a trainer can give enough care and attention. Despite this, heroes in the Adventures manga often own more than six Pokémon at a time, keeping several in the PC and rotating them as needed.
This means that trainers are either not taking care of all of their Pokémon properly, or they may be taking care of more than six, regardless of Oak's analysis.
The Pokémon anime is inconsistent about what happens when a trainer catches the seventh Pokémon. In the thirteenth episode of the original series, "Mystery at the Lighthouse," Ash captures his seventh Pokémon, a Krabby, and Ball teleports to Professor Oak's laboratory.
In contrast, the black and white episode "Siedel and Berg in Pinwheel Forest!" In 2011, when Ash catches his seawall, the ball contracts and locks in with a red light, allowing Ash—who is eventually making another Pokémon game appearance after 22 years—until he takes one of his Pokémon. Unable to use Sevdl until one has accumulated.
On the other hand, in the Pokémon Journey episode "A Snow Day for Searching!" Goh is able to keep up to seven Pokémon in his party without any problems.
In the mainline game series, the six Pokémon limit is a hard rule for team size; The player cannot have more than six Pokémon at a time. Even so, there are a few instances of this rules game lurking.
The collected Zygarde cells in the Sun and Moon also do not count, but a complete Zygarde does. The ride of the Pokémon called in Legends: Arceus or Sun and Moon also does not affect the number of Pokémon the player has.