Product : Citrix, XenServer/6.5, Commercial Edition
Feature : Max vCPU per VM, VM Config , Hypervisor
Content Owner:  Roman Macek
Summary
16 (Win) / 32(Linux)
Details
You can only specify up to 16 vCPUs using the XenCenter GUI (you can increase this for Linux guests using the command line e.g. xe vm-param-set VCPUs-max=32 uuid=...). Actual numbers vary with the guest OS version (e.g. license restrictions), see http://bit.ly/1ELdzpv for details.