Each repository has its own database. The same goes for Audit Trail (each repository you set up in the reporter creates a corresponding SQL database). One single SQL database serves one installation of Workflow, regardless of how many repositories are used in Workflow on that server.

The number of database that can live on the same server depends on the server specs and the load on each one. Microsoft might be a better source of documentation on that.

As a very basic rule, Audit Trail is very SQL-intensive if you run reports a lot. We usually recommend putting it on its own SQL instance so it does not affect the performance of Laserfiche. Depending on how much data you plan on keeping around for reports, that separate instance could run on SQL Express.