Proxmox for 40k vm would be surprising also veeam support Proxmox.

I'd would assume that this is not a monolithic cluster of 40k vm's but at least tens of clusters. Which puts it in the realm of capabilities of Proxmox.

	▲	nick__m 4 hours ago \| parent \| next [-]
		Before my vacation we (3 colleagues and myself) completedan 8 months long migration (coordination with stakeholders is longer and more complex than migrating a 192TB VM !!!) to 6 proxmox clusters so 20 to 40 clusters for 40k is certainly possible but imo it would be unwieldy.
	▲	beniihana 2 hours ago \| parent \| prev [-]
		I’d guess thousands of clusters. They have over 3k retail stores in the UK, so that could be a 2-3 node cluster in every one. I’ve worked with a few major US grocers on very similar projects (some hardware only refreshes and one VMware to HyperV/Azure Local migration).