Top N Report does complete creating report in UMP when choosing a large set of server group (More than 1000). Initially no reports were working but after following KB44786, reports are working for smaller set of servers (less than 1000 servers).
Release : 20.1
Component : UIM - UMP_REPORTS
The Top N Reports are not intended to be a full report but only "Top" consumers, e.g. highest users.
If you require a report showing ALL devices in a large group or a larger number then it will be better to create a custom report (please be mindful of the load the database would run into querying and sorting top 6000 devices from a very large number of devices - this may lead to performance issues at the database level). Top N reports is specifically intended for 'quick' views of the highest consumers.
You can create custom reports using the this techdocs: CreateAdhocReports.