The number of people whom one person can supervise effectively is known as:

The concept being described is known as the span of control. This term refers to the optimal number of subordinates a supervisor can effectively manage and supervise. A well-defined span of control facilitates clear communication, efficient management, and effective delegation of tasks, which enhances teamwork and productivity within an organization.

In the context of firefighting and emergency services, maintaining an appropriate span of control is crucial, especially in high-stress situations where effective supervision can directly impact safety and operational efficiency. A span that is too wide may lead to insufficient guidance and support for subordinates, while a span that is too narrow can result in micromanagement and inefficiencies.

The other concepts, such as division of labor, functional supervision, and chain of command, address different aspects of organizational structure and management, but they do not specifically define the number of individuals one person can supervise effectively. Therefore, span of control is the most applicable term in this instance.