Context: A server farm or server cluster is a collection of computer servers maintained by an organization to supply server functionality far beyond the capability of a single device. Modern data centers are now often built of very large clusters of much simpler servers, and there is a collaborative effort, Open Compute Project around this concept.

Question: a large collection of servers is called a(n)?
Steam of consciousness below:
The relevant sentence in the passage is: A server farm or server cluster is a collection of computer servers maintained by an organization to supply server functionality far beyond the capability of a single device.
So, the answer is A server farm or server cluster.