How to scale up and down nodes automatically?

That's correct, to scale you would change the number of nodes in the nodeSet (or potentially replace a nodeSet to increase storage/node). We do not have support for the HPA or another autoscaling mechanism at this time. Unfortunately adopting the HPA model for Elasticsearch is challenging, as it expects there to be a single underlying deployment/sset/etc. In this case multiple ssets (can) make up an Elasticsearch resource. We hope to have a better autoscaling story in the future, but for now you would need to create something tailored for your use case.