Hi,
I can see sCategories class returns $category array which has all info about category but not article count.
That’s right. This is what you have to add to the array In this article you can read about how to find extension points: Best practices of Shopware plugin development
In your case you need an after hook on the sCategories::sGetCategories() method, to extend the array which the method returns. Unfortunately there is no event, you can use. After you added the new value, you can use it in the themes/Frontend/Bare/frontend/index/sidebar-categories.tpl template file.
Best regards
Michael Telgmann