A service that might appear to be a single server to external clients, but actually represents many servers living behind it, is known as a

A reverse proxy.

Explanation:

A service that might appear to be a single server to external clients, but actually represents many servers living behind it, is known as a reverse proxy.

In Computer Networking, a single server can be configured to appear as the endpoint for many other servers acting behind it through the use of a reverse proxy.

Hence, a reverse proxy is a type of server that is placed right in front of other servers such as a web server and forwards or routes the user's (client) requests to those servers sitting behind it. When properly configured, reverse proxy helps to ensure security, reliability and performance of a network.

i think the answer would be a