The following has just been noticed in the ‘ListView’ subpanel:
In version 7.x, the number of columns was much more flexible. In version 8.x, the number of columns for the list view can be set globally for the modules in the file ./config/services/module/listview/column_limits.yaml.
However, I have not found a way to set the number of columns in the subpanel ListViews. Currently, only 6 columns are displayed.
According to the Angular code the max columns used for sub panels is the same calculator as the main list view:
listview_column_limits
It uses the data in the yaml files and switches depending on whether the sidebar is active or not. You add remove actual data using the metadata/subpanels/ for the module that is being listed (and selected as a part of the relationship).