I agree that the number of sessions is something that belongs and is/should be exposed via monitoring, however that implies at least a couple of changes which would "complicate things":
1) expose `RepositoryMonitor` from `Repository` in `modeshape-jcr-api`
2) update/enhance the `Window` functionality with the semantic of _now_, because in this case we really need the current information, not a history

If you prefer, I can look at these changes, but they would make this perhaps more complicated than it needs to be.