Is Workflow Server SaaS compliant? Can it run in the cloud?

Yes, Workflow Server can be easily deployed and can run on any cloud environment capable of running .NET Core.

Workflow Server’s workflows are isolated from one another. Hence, you can execute them on different servers, having as many instances of Workflow Server as you need, whereas each Workflow Server’s instance can use a separate database for Persistence.

You can read more about scalability in multi-server mode here.