Remove unused Finder tags
To view all the available tags in Finder, follow these steps:


Enable display of tags by going to Finder → Preferences → Sidebar → Tags → Recent Tags.





In the Finder menu, enable show Sidebar and Status Bar if they are hidden. 
Under Tags section in the Finder Sidebar, click on All Tags.... This will display a list of all the available tags (pre-created + custom) sorted alphabetically.
Click to select a tag from the list. The count of files/folders associated with the tag is shown in the Status Bar.





To delete a tag, right click on it and select Delete Tag "Tag_Name"... from the context menu.






For more information on tags in Finder, refer to the Apple support document, macOS Sierra: Use tags to organize files.