Here is the official word from Microsoft: In particular, this paragraph:
KMS is a lightweight service that does not require a dedicated system and can easily be co-hosted on a system that provides other services. With KMS, you can complete activations on your local network, eliminating the need for individual computers to connect to Microsoft for product activation.
A KMS host key is used only to activate the KMS host with a Microsoft activation server. A KMS host key can activate six KMS hosts with 10 activations per host. Each host can activate an unlimited number of computers. If you have an existing machine configured as Windows KMS* host, you will need to enter and activate the Office 2010 KMS host key before the KMS host can activate Office 2010, Project 2010, and Visio 2010. If you need additional KMS activations so you may activate more than 6 KMS hosts, find the telephone number for your Microsoft Activation Center to activate your KMS host.
KMS requires a minimum number of either physical or virtual computers in a network environment. These minimums, called activation thresholds, are set so that they are easily met by enterprise customers. For computers running:
Windows Server 2008 and Windows Server 2008 R2 you must have at least five (5) computers to activate.
Windows Vista or Windows 7 you must have at least twenty-five (25) computers to activate. These thresholds can be a mix of server and client machines to make up the threshold number.
Office 2010, Project 2010 and Visio 2010 you must have at least five (5) computers to activate. If you have deployed Microsoft Office 2010 products, including Project 2010 and Visio 2010, you must have at least five (5) computers running Office 2010, Project 2010 or Visio 2010.
Here are some more reference materials to assist you:
For Activation thresholds, please see the Volume Activation Planning Guide.
For Office 2010, Project 2010, and Visio 2010 please see Volume Activation Quick Start Guide for Office 2010, and Overview of Volume Activation for Office 2010.
Read more details about the product activation process and the specific products that use Volume Activation.
*Only Windows Server 2003, Windows 7 volume editions, and Windows Server 2008 R2 are supported as Office KMS hosts.
This one is a bit confusing. Is it 6 KMS servers or 60??? Here is a bit more details from Sean Metcalf
A KMS key is used to activate only the KMS host with a Microsoft activation server. A KMS key can activate up to six KMS hosts with 10 activations per host. Each host can activate an unlimited number of computers. If you need to activate more than six KMS hosts, contact your Volume Licensing Service Center (http://go.microsoft.com/fwlink/p/?LinkId=184280), and state why you must increase the activation limit.
In other word, the answer is 6.