This will return the number of items in the list. Please note that you can of course either call this method directly or implicitly by using "count($list)" in PHP or "list | length" in Twig, respectively.
{% set list=Node::queryAll({category:'sports'}) %}
The "sports" category has the following {{list | length}} items :
<ul>
{% for item in list %}
<li>{{ item.$title }}</li>
{% endfor %}
</ul>