Add a SiteSetting to configure how many topics are in the email digest.

app/mailers/user_notifications.rb

@@ -42,7 +42,7 @@ class UserNotifications < ActionMailer::Base
     @last_seen_at = I18n.l(@user.last_seen_at || @user.created_at, format: :short)
 
     # A list of topics to show the user
-    @featured_topics = Topic.for_digest(user, min_date, limit: 20, top_order: true).to_a
+    @featured_topics = Topic.for_digest(user, min_date, limit: SiteSetting.digest_topics, top_order: true).to_a
 
     # Don't send email unless there is content in it
     if @featured_topics.present?

config/locales/server.en.yml

@@ -838,6 +838,7 @@ en:
 
     allow_uploaded_avatars: "Allow users to upload their custom avatars"
     allow_animated_avatars: "Allow users to use animated gif for avatars. WARNING: it is highly recommended to run the avatars:regenerate rake task after changing that setting."
+    digest_topics: "The maximum amount of topics to display in an email digest"
     digest_min_excerpt_length: "How many characters we're aiming for for each post in the email digest"
     default_digest_email_frequency: "How often users receive digest emails by default. They can change this setting in their preferences."
     default_external_links_in_new_tab: "Open external links in a new tab. Users can change this in their preferences."

config/site_settings.yml

@@ -253,6 +253,7 @@ email:
   email_time_window_mins: 10
   email_posts_context: 5
   digest_min_excerpt_length: 100
+  digest_topics: 20
   default_digest_email_frequency:
     default: 7
     enum: 'DigestEmailSiteSetting'