At work, we just went over 3000 VMs. 
This is with a relatively small team, thanks to some innovative automation of provisioning and management.
Interestingly, it took us just 8 months to go from 2,000 to 3,000 VMs. Hopefully this rate of growth (sprawl?) will slow down a bit soon. We did actually go down a bit recently, as we cleaned up some no longer used machines. (Note that the green in the graph is from VMs on old ESXi 4.1. The remaining 4.1 VMs now are desktops in View (virtual desktop infrastructure.) We are just about to move these desktops into ESXi 5.5 Update 3 replacement cluster with Horizon 6.2, so the green will soon go away entirely.
