I keep hearing about "ideal" number of direct subordinates, which is 5 to 7. It's also called "Span of control"
Span of control, also known as span of management, is a human resources management term that refers to the number of subordinates a supervisor can effectively manage. [Source]
The concept and the number dated back to 1922:
In studies of British military leaders, Hamilton found that they could not effectively control more than three to six people directly. These figures have been generally accepted as the "rule of thumb" for span of control ever since. [Source]
Was it tested or topic additionally researched in Information Technologies era ?
