It was 3 people who were replaced for the making of a list.

The number 50 was what Doctorow presumed was the entirety of the department that could potentially have been replaced by AI, of which the making of this list had been only one of that department's overall tasks.

At 3 interns per article, having 30 interns working on 10 simultaneous articles at any given time seems like reasonable output for an online zine.