Listing categories with post count in a nav?
Within you nav loop, you would use the method total() with an craft.entries ElementCriteriaModel, to only count the entries that are related to the category you're looping. Pass the CategoryModel to the relatedTo parameter.

{% nav category in craft.categories.group('articleCategories') %}

    {# Get the count of entries related to `category` #}
    {% set entryCount = craft.entries.relatedTo(category).total() %}

    {{ entryCount }}

{% endnav %}