In the Champion menu, there are 2 Limits.
The first limit is for Champions. It is visible to every player. It is determined by the number of slots the player has unlocked. The maximum number of Champion slots is 440. Only Champions are added to these slots. Geists are not counted there.
The second limit for Geists is not indicated and cannot be tracked or increased. The maximum number of Geists every player can have is 300. Only Geists are included in this limit. If a player tries to claim Geists exceeding this limit of 300, they will not be added to the collection. To successfully claim them, the player needs first to clear up some Geists' space by sacrificing some Geists to upgrade their Champions or other Geists.