Can the Rancher Management cluster scale more than three nodes? Can it work in multiple datacenters? I am wanting to scale to three nodes each in three datacenters.

Rancher can run in any Kubernetes cluster. Getting Kubernetes (etcd in particular) to happily run across three datacenters might be an interesting task. You’d need to keep the datastore latency down, so they would probably need to be pretty close to each other.

to use AWS terminology, three zones (which are generally 3 different buildings) is ok, 3 regions (totally different regions) is not

I will let you know how it goes.