During the VMware vCenter Server 5.1 installation there is an option to check a box that says “Increase the number of available ephemeral ports.”
The description reads:
If your vCenter Server manages hosts on which you will power on more than 2000 virtual machines simultaneously, this option prevents the pool of available ephemeral ports from being exhausted.
Is the total number of virtual machines per host or per vCenter Server? This wording is unclear, but I suspect the latter (e.g. per vCenter) as that would be a awfully large host to be able to run 2,000 virtual machines.