At that point you probably need a cluster of k8s clusters, no?
For anyone unfamiliar with this the "official limits" are here, and as of 1.32 it's 5000 nodes, max 300k containers, etc.
https://kubernetes.io/docs/setup/best-practices/cluster-larg...
Yes this is what I'm referring too. :)
Maintaining a lot of clusters is super different than maintaining one cluster.
Also please don't actually try to get near those limits, your etcd cluster will be very sad unless you're _very_ careful (think few deployments, few services, few namespaces, no using etcd events, etc).