I’m not sure what you’re looking for: Flowable is an open-source library and doesn’t require licensing. So I assume you want to add a licensing check in some way? If so, that would need to be added in custom code you add on top of the API’s of Flowable that you’re calling.
If you are using the flowable engines or flowable via the REST API, as far as I know, there is no way to track the active users. The best way would be for you to have an application than embeds flowable and do you user counting code in the application outside of flowable.
YOur application would only call the flowable APIs when needed and your application would keep metrics of what user initiated the calls so that those users can be counted as active.