The badge number on the subscription can change meaning depending on some settings.
Automatic Downloads, Streaming OFF
This is the default, the badge indicates the number of downloaded, unlistened shows
Automatic Downloads, Streaming ON.
The badge indicates the number of unlistened shows
Manual Downloads, Streaming ON or OFF
The badge indicates the number of unlistened shows, badge turns blue.
An exclamation point instead of a number means that the podcast could not be refreshed at the last try. Ensure you have a good internet connection and "pull to refresh".