Every monitor has its own set of workspaces. Workspace 4 on display A is not the same as Workspace 4 on display B is not the same as Workspace 4 on display C.
Functionally you can focus any workspace on any monitor, and ids do not change when doing that. You can set things up both ways, but it's actually easier to just have a key focus workspace x on the current monitor than to lock workspaces to specific monitor.