[throw0101a]

> Probably Proxmox. Veeam support is relatively new.

As a sysadmin of Proxmox, I do not see how it can scale to 40k VMs. The Proxmox folks themselves have seen "~24" nodes in a cluster (theoretical support is higher), so you'd probably need a lot of clusters for 40k:

* https://forum.proxmox.com/threads/proxmox-with-48-nodes.1746...

For such a size (and sticking strickly with open source), XCP-ng could be an option, or OpenStack. In the closed source space, Nutanix.

As of 2021, CERN had 35k instances/VMs in their OpenStack implementation:

* https://superuser.openinfra.org/articles/scaling-bare-metal-...