Many WordPress users search around the Customizer and grow impatient when they can’t find this setting.
For some reason, this option not yet available in the Customizer, but it is readily found in the Reading Settings menu in the admin dashboard.
When you update the number of posts displayed on the blog, it affects every page of the blog. In addition, this setting is reused for your post archives. This includes category, tag, author, and date archives.