Policy
Number of folders in the Recent Folders list
This policy setting specifies the number of unpinned entries displayed in the Recent Folders list that appears when users click Open or Save As on the File tab in Backstage view. If you enable this policy setting, you can specify the number of unpinned entries to be between 0 and 20. If you set the number to 0, all pinned and unpinned entries are hidden. If you disable or do not configure this policy setting, a maximum of 5 unpinned items are displayed in the Recent Folders list. Note: If you want to prevent items from being added to the Recent Folders list entirely, you can enable the "Do not keep history of recently opened documents" Windows policy setting.
5b04c07440ad L_SetNumberOfPlacesInTheRecentPlacesList Registry
Copy registry mappings
HKCU\software\policies\microsoft\office\16.0\publisher\place mru\max displayPolicy notes
This policy setting specifies the number of unpinned entries displayed in the Recent Folders list that appears when users click Open or Save As on the File tab in Backstage view. If you enable this policy setting, you can specify the number of unpinned entries to be between 0 and 20. If you set the number to 0, all pinned and unpinned entries are hidden. If you disable or do not configure this policy setting, a maximum of 5 unpinned items are displayed in the Recent Folders list. Note: If you want to prevent items from being added to the Recent Folders list entirely, you can enable the "Do not keep history of recently opened documents" Windows policy setting.