We're running a migration from VMware to Red Hat Openshift Virt for one customer right now which is over 150,000 VMs. That's a big one, but five figure number of VMs is very typical.

I gave a 5 min lightning talk about this a couple of years back: https://pretalx.com/devconf-cz-2024/talk/SN93LG/

People talking here about Proxmox make me laugh.