Hello Pawel Markowski
Welcome to Microsoft Q&A Platform, thanks for posting your query here.
Adding to previous response from Micheal, you can also try Azure Resource Graph Explorer to get the reservation details.
Open Resource Graph Explorer from the Azure portal. Run below query in the editor:
where type =~ 'microsoft.compute/virtualmachines'
| summarize count() by tostring(properties.extended.instanceView.reservation.id)
The query result will show the number of VMs included in each reservation. You can click on each reservation ID to see the details of the VMs included in that reservation. You can also download CSV as shown in below screenshot.
Hope this helps.
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