Capacity Planning - The Desktop Delivery Controller is responsible for:

• Authenticating end users against Active Directory. 
• Enumerating available resources.
• Creating registrations for newly started virtual desktops.
• Maintaining an active heartbeat with online virtual desktops.

XenDesktop functions with a single controller. During rapid concurrent or simultaneous end user logons, where hundreds or thousands of end users connect to the environment in a short amount of time, a bottleneck, or traffic congestion, can occur in the Desktop Delivery Controller. This bottleneck can result in long logon times or even denied service. By separating controller functionality across multiple Desktop Delivery Controllers, the overall XenDesktop farm supports more virtual desktops and responds faster.
To provide fault tolerance, configure XenDesktop implementations with redundant Desktop Delivery Controllers. For environments that are expected to host more than 1,000 virtual desktops, separating the controller functionality across a minimum of five Desktop Delivery Controllers is recommended. The role designation across the five servers would include one master Desktop Delivery Controller, two XML controllers, and two Web Interface servers. For more information about distributing the functions across multiple Desktop Delivery Controllers, see Citrix article CTX126190

Sizing Virtual Desktops on a Single Host Server - You can determine the number of virtual desktops that a host server can support by analyzing the physical hardware specifications of the host server and the resource utilization of each virtual desktop. As a guideline, the physical hardware specifications of the host server should be able to support the average resource utilization of the virtual desktops plus 20 percent to account for periods of peak usage. For more information about sizing virtual desktops on a single-host server, see Citrix article CTX124086
A host server has the following physical hardware specifications: 

• Four 3.2 GHz processor cores, which equals 12.8 GHz of processing capability 
• 16 GB RAM
• 180 GB disk space
• 50 MB/sec of disk I/O
• 2 Gb network interface

The following table displays the maximum number of virtual desktops based on the utilization of each resource.

Host Server Resources (Reduced by 20%)	Determined Resources of each Virtual Desktop	Possible Number of Virtual Desktops
CPU: 10.24 GHz Memory: 12.8 GB Disk Space: 144 GB Disk I/O: 40 MB/sec Network: 1.6 Gb	Peak CPU: 795 MHz Peak memory: 604 MB Peak disk space: 5.9 GB Peak disk I/O: 428 KB/sec Peak network: 2420 KB/sec Average CPU: 351 MHz Average memory: 524 MB Average disk space: 5.4 GB Average disk I/O: 216 KB/sec Average network: 202 Kb/sec	12 21 24 93 661 29 24 26 185 7920

Note: The total number of supported virtual desktops should be equal to the lowest possible total number of virtual desktops for any of the average resources. In this example, the virtualization hardware can only support 24 virtual desktops based on the average memory requirements. 

• The physical hardware specifications of the virtualization server were reduced by 20 percent to verify that the host can support the average resource utilization of the virtual desktops and account for periods of peak usage. 
• This example assumes that each virtual desktop has its own virtual hard disk supported by the virtual host server.
• Depending on the tools used to gather performance metrics, it may not be possible to gather the resource utilization results for each of the resource sets shown in the example. In that case, data collection that includes the desktop operating system with the applications running as a whole can work as a substitute method.