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.

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.

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).