Hello!
Single-player games can be created.
Each package you build with the Creator Toolkit requires a project configuration. The project configuration for a space has a setting (Server Instancing) to limit the number of people in an instance. See more here: https://docs.spatial.io/project-configuration
To turn off Spatial GUI systems, such as chat, you'd use the SpatialBridge's coreGUIService: https://cs.spatial.io/reference/SpatialSys.UnitySDK.ICoreGUIService