Hi @Amir Golan
Welcome to Microsoft Q&A! Thanks for posting the question.
Unfortunately the monitoring the alert condition are only at the Entity level i.e. Queue/topic therefore you will not see any condition at the subscription level when you create the alert rule as mentioned in the metric section for Metric Name "Active Messages" and the Dimensions are "Entity name".
Please feel free to raise a feature request on UserVoice to support this, if this is something that you would want to see in service bus monitoring.
Alternatively, if feasible you need to create the middleware to monitor the subscription active message count using service bus SDK and use custom event grid for event based scenario or middleware have the logic for further processing events based on your business logic.