[Answer]-Get count on filtered subqueries in template

You’re right that you’ll have to do this in the view. You can use annotation to get the number of filtered articles for each subcategory:

from django.db.models import Count

def my_view(request, other, arguments):
    ...
    subcategories = category.sub_category_set.filter(tag__tagname='xyz') \ 
                            .annotate(num_articles=Count('article__id'))
    ...

Pass subcategories to your template context, and in your template you can do this:

{% for subcategory in subcategories %}
    {{ subcategory.name }} -- {{ subcategory.num_articles }}
{% endfor %}