On an SSRS instance with a remote ReportServer db, where is the processing done? Should the SSRS instance vm get additional cpus and memory or the sql instance where the ReportServer db is? The reports' target sql instances are fine cpu and memory wise. Thanks!
Is the ReportServer db/server just serving up report and folder info/configurations, or is the loading of a report running through the that server too?
It's mostly just for storing and retrieving meta-data information about the reports, folders, and SSRS instance, yes. Nothing particularly heavy runs in this database instance.
The only thing I'd bear in mind is report subscriptions. Those get scheduled with a SQL Agent Job here. The more schedules there are and the more frequently they fire off, the more resource contention there is. Five SSRS database instances hosted on the same SQL Server sounds like a decent amount, but mainly only if there's a lot of report subscriptions running.
I figure the SSRS instance server is handling the web portal and reportserver processing?
Yes, that's correct. The actual rendering of the report itself (which can be heavy depending on how you designed it) happens on this server.
As far as dividing up your hardware resources, there's no one specific answer here and would require testing with your actual reports and report instances. Just keep in mind the above information when provisioning. Generally I've never needed more than 4 CPUs for the SSRS instance itself on fairly busy report servers, but that was for a single instance.
Also be mindful of licensing, which in some cases would require SQL Server licensing for both the report instance in addition to the database instance.