Well, it's not always as simple as 'how many systems' even when you're talking about just an operating system. There's licensing based on cores and/or sockets. Then there's licensing for virtual machines.

Oracle was really nasty in some of the licensing scenarios, IIRC. If you ran an Oracle DB on an unlicensed platform in a VM (such as VMWare), then when you got audited, you needed to pay for how many cores/sockets were in the physical server, rather than just how much CPU your VM was using.

Is there evidence of Red Hat ever doing that kind of gotcha licensing?

I don't know of any, but that isn't to say there isn't any. Oracle on the other hand is widely known for this practice.