Views makes it easy to include a node's comment count as a field, but what about when you aren't using fields...when you display content, instead?
You could write a preprocess function, but why code when you don't have to? Use twig!
The comment class includes a method count(), but that method is not exposed to twig. Luckily, an alternative is:
{{ node.comment.comment_count }}
That's all there is to it!