Can't speak for GP, but it really will vary on workload. How much data is in each DB, how much workload is write vs read. In a mostly read scenario (say a blog), I'd be surprised if you couldn't handle 100k simultaneous users on a fast drive, more with a good caching strategy.

For modest read/write, should be able to get 1000-10k users without much issue.

A little advice would be to ensure logging is to a separate db/drive from the main data if you are logging changes, etc.

The only down side is distribution with High Availability, but so many scenarios don't actually demand the extra overhead for true HA.