Thank you for your question! That’s right, generally one character and one key per scene but a player can play more of each if they want to within the narrative.
Keys replace the setting elements found in other games based on Belonging Outside Belonging. In those games, setting elements are all the pieces of a game usually controlled by a Gamemaster. They are aspects of the setting that the characters can interact with.
Anything can be a Key. The trees in a scene, the sound of a tractor tumbling along the country path, a flock of birds.
The difference between Character and Key is that you own your Character, only you decide what they can do. Nobody owns a Key, and anyone can collectively use any key to drive the narrative.