Find the nth top-level category in a nav tag
Maybe you can add in a little counting magic of your own:

{# before the nav tag #}
{% set topLevelIndex = 1 %}

{# inside nav loop #}
{% if category.level == 1 and topLevelIndex == 10 %}
    &lt;li&gt;Zoiks! That‘s a lot of categories!&lt;/li&gt;
{% endif %}
{% if category.level == 1 %}
    {% set topLevelIndex = topLevelIndex + 1 %}
{% endif %}


This code hasn't been tested...