Just found this PDF online of the configuration limits for Citrix XenServer 6.0.
Citrix recommends using the following configuration limits as a guideline when selecting and configuring your virtual and physical environment for XenServer. The following information represents tested and recommended configuration limits that are fully supported by Citrix for XenServer.
You can download the PDF here from the Citrix website.
More whitepapers about the XenServer 6 product can be found here.