<span>You have {{products|length}} products</span>
You can also use this syntax in expressions like
{% if products|length > 1 %}
jinja2’s builtin filters are documented here; and specifically, as you’ve already found, length (and its synonym count) is documented to:
Return the number of items of a sequence or mapping.
So, again as you’ve found, {{products|count}} (or equivalently {{products|length}}) in your template will give the “number of products” (“length of list”)