How much virtual linux you can boot?
Computer security scientists from Sandia National Laboratories in Livermore, Calif. show how to run more than a million Linux kernels as virtual machines. They use 4480-node high-performance computer cluster. It is about 225 virtual machines on each real one.
They did this not only for record number of VM but also to create tools to investigate/model behavior of botnets with high number of hosts.