Thanks for your question, @saggl !
With the decentralized approach that will be implemented in this project (https://github.com/corona-warn-app/cwa-documentation#about-this-project) this is not possible, as the number of contacts is not exposed through the API (see also #32). And even if this information is available through the OS, you would only know the number of exposure events and not be able to tell how many unique users those belonged to.
I don't think that this is possible to implement due to privacy-related issues that would be caused by such a feature.