If you consider that a “Robot” itself is technically a Windows user, each of them would consume an user CAL.
On the top of that, You will need to consider the users accessing the Orchestrator front-end as well.
There is the eventuality you could consider your infrastructure as a Device CAL model (I would contact with confirm that with Microsoft, showing them your infra diagrams) but that would be only interesting if you have only few bots and one or two persons maintaining/developing on them.
using any CAL model would prevent you to scale (scaling is usually what you your are looking for using when UiPath), that’s why the vast majority of clients use a per core model.
Cheers