I don’t think there’s a .Page variable with the replies count available, unfortunately. You could call the JSON API or Conversation.js from Hugo to figure it out, but that’s a bad idea™. It would result in long build times for your blog and stress the network unnecessarily. Also, if someone comments on a post after the blog has been built, the conversation won’t show until the next build.
As fetching the conversation already happens client side, I would do the <details> dance with JavaScript as well.