base.html

{% load core_utils %}

<ul id="tags" class="nav nav-tabs nav-stacked">
    {% for tag in tags %}
        {% if tag_full_list|list_count:tag %}
            <li>
                <a href="{% url "post:posts-tagged-related" tag.slug %}">
                    {{ tag.name }} <span class="label label-info">{{ tag_full_list|list_count:tag }}</span>
                </a>
            </li>
        {% endif %}
    {% endfor %}
</ul>

context_processors.py

# coding: utf-8
from taggit.models import Tag
from core.models import Post


def active_post_tags(request):
    # Lista tags referentes aos posts ativos
    posts = Post.objects.ativos()
    active_tags = [post.tags.all() for post in posts]

    tag_full_list = [t for tag in active_tags for t in tag]
    all_tags = Tag.objects.all()
    tags = [tag for tag in all_tags if tag_full_list.count(tag)]

    return {'tags': tags, 'tag_full_list': tag_full_list}

core_utils.py

from django import template

register = template.Library()

@register.filter
def list_count(value, arg):
    return value.count(arg)