OpusK is looking for a way to count topics in sub-categories as an Admin. jomaxro suggests two methods: trying to delete a sub-category to see the topic count in the error message, or writing a Data Explorer query. OpusK finds the first method helpful but wishes the topic count was visible at a glance on the Admin Page. jomaxro provides two Data Explorer queries: one to get the topic count for a specific category, and another to list all categories and their topic counts. OpusK is grateful for the help and finds the queries useful for future reference.
I do not need to display it to anyone. I just want to see how many topics are in my sub-categories by Admin.
While it’s easy to see the top-categories, we’re not looking for those features in sub-categories.
Are these features basically missing?
If it is not, I would appreciate it if anyone let me know how it works 
The quickest way is likely to open the sub-category edit page and try to click delete. It will fail as there are topics, but the message will give you the number of topics. Alternatively, you could write a simple Data Explorer query.
Oh, that’s a very simple way!
However, I can not see it at a glance in Admin Page .. 
Anyway, it seems to be good to use as it is urgent.
Your answers are a big help because I am in a hurry right now.
Here are some quick Data Explorer queries in case you need to do this more in the future.
Query to get topic count for one category (or subcategory).
-- [params]
-- null string_list :category_name
SELECT topic_count FROM categories
WHERE categories.name = :category_name
Query to list all categories (and subcategories) and their topic counts:
SELECT name, topic_count FROM categories
Wow!
It will help me a lot in the future.
Thank you for your kind reply