Unfortunately badge management is not straightforward in a pub/sub architecture. If you use tags to indicate individual users then you can keep your counter in your backend and then add it in your notification when sending to your hub.
In you are using tags as interest group, then things are not straightforward, and they depend on what you want that count to be. For some platforms (i.e. iOS and ANdroid) you can code some client code that keeps a personalized counter. Some other times however, one has to avoid keeping personalized counters and resort to use generic badges like "!" in Windows 8.