DiscourseAddNewPostsToSummary adds the number of new posts to summary/digest email in the 3rd position (usually where new users is).
Follow Install a Plugin
how-to from the official Discourse Meta, using git clone https://github.com/literatecomputing/discourse-add-new-posts-to-summary.git
as the plugin command.
Plugin is enabled by default.
If you have issues or suggestions for the plugin, please open an issue at github.