For any one journal, very few, maybe even fractions of a headcount per journal, but for the thousands of journals out there spanning dozens of disciplines and hundreds of specialties, it adds up. If you want to make the end-all-be-all magic journal of all-topicness and maintain a respectable level of quality, you're going to need quite a few SMEs policing the submissions.
There's millions of scientific papers published annually - you need people to process all of that information and moderate peer reviews.