This will depend on your data, hardware and query requirements. I would recommend looking at the following resources:
https://www.elastic.co/webinars/using-rally-to-get-your-elasticsearch-cluster-size-right
https://www.elastic.co/elasticon/conf/2016/sf/quantitative-cluster-sizing
https://www.elastic.co/webinars/optimizing-storage-efficiency-in-elasticsearch
https://www.elastic.co/elasticon/conf/2018/sf/the-seven-deadly-sins-of-elasticsearch-benchmarking