@user-MygFNHEclO There's no way to do that via a Smart Group (or at least none I've ever figured out). If you host your Jamf Pro instance on-prem, and have access to the MySQL database, you can do it by querying the database directly. Then there's the brute force method of generating a .csv report of all systems listing the Mac serial number and enrolling user then dropping that into Excel to identify users who appear multiple times.
I suppose the better question is why are you wanting/needing to do this?
In most cases I have seen its upper brass is wanting reports to see who has multiple devices. You can do that with an advanced search and exporting it with devices and user accounts visible, then just sort the data in excel/numbers. Beyond that it would probably be a custom SQL query as @sdagley suggested, but JAMF would not be able to do anything with that data (ON PREM ONLY). I am sure JAMF API could also export that information. But JAMF itself cannot do anything with this information.
I was looking for a solution to the same problem and accidentally ended up in the jamf pro user area. there you can see how many devices are assigned to a user and sort them according to the highest number. I don't know if that will help you as well. but that was sufficient for my purposes