User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\groups
This policy setting controls the maximum number of groups that are accessible through the Navigation Pane in Outlook for Windows. If you enable this policy setting, only the number of groups set in the Groups Count will be shown in the Navigation Pane. If you don’t enable this policy setting, the maximum number of groups displayed defaults to 1000.