Hosting SSRS databases in Azure SQL Managed Instance

Michiel_Klaassen , you can choose among different licensing options, depending on your preference and whether you enjoy the License Mobility benefit of Software Assurance, or not.

If you search the Azure Marketplace for available Azure VM images containing Reporting Services, you will notice two types of images:

One type with name starting with {BYOL} - that's where you use your existing licenses

Another type without {BYOL} prefix - those are Pay-As-You_Go model with license included in the price.

Each of the types also offers two variants: Reporting Service only, and Reporting Services + SQL Server database engine.

If you want to host your two system databases of SSRS on Azure SQL Managed Instance, you don't need SQL Server database engine on the Azure VM.

If you want to host your two system databases of SSRS on the same Azure VM, you'll need an image with SQL Server database engine.

While browsing the available Reporting Services images, you can click on the Plans tab, and see more details regarding licensing.

The second part of the following blog post also describes the licensing options. You can see the Plans tab on the second screenshot in the article:

Azure SQL VM Automatic Registration and Reporting Services Images (microsoft.com) 

I hope this helped.