Each vm belongs to a single cluster. The best way to maximimize resource utilization is to size the vms according to the tenant load, generally you want to minimize the number of vms, as each of them add some overhead, but you don't want to waste unnecessary resources. You must have a minimum set of vms in each cluster to ensure high availability.

Automating the provisioning of an entire new cluster is very easy, at least for some people with the required resources. But even for a small shop it is possible to automate it using stuff like Jenkins and Ansible. It is still relatively sophisticated stuff. It is not rocket science but not an elementary school project level either.