Clarify recommendations about the number of ES shards

doc/integration/elasticsearch.md

@@ -222,7 +222,7 @@ The following Elasticsearch settings are available:
 | `Pause Elasticsearch indexing`                        | Enables or disables temporary indexing pause. This is useful for cluster migration/reindexing. All changes are still tracked, but they are not committed to the Elasticsearch index until resumed. |
 | `Search with Elasticsearch enabled`                   | Enables or disables using Elasticsearch in search. |
 | `URL`                                                 | The URL to use for connecting to Elasticsearch. Use a comma-separated list to support clustering (e.g., `http://host1, https://host2:9200`). If your Elasticsearch instance is password protected, pass the `username:password` in the URL (e.g., `http://<username>:<password>@<elastic_host>:9200/`). Special characters in the username or password should use [percentage encoding](https://en.wikipedia.org/wiki/Percent-encoding). |
-| `Number of Elasticsearch shards`                      | Elasticsearch indexes are split into multiple shards for performance reasons. In general, larger indexes need to have more shards. Changes to this value do not take effect until the index is recreated. You can read more about tradeoffs in the [Elasticsearch documentation](https://www.elastic.co/guide/en/elasticsearch/reference/current/scalability.html). |
+| `Number of Elasticsearch shards`                      | Elasticsearch indexes are split into multiple shards for performance reasons. In general, you should use at least 5 shards, and indexes with tens of millions of documents need to have more shards ([see below](#guidance-on-choosing-optimal-cluster-configuration)). Changes to this value do not take effect until the index is recreated. You can read more about tradeoffs in the [Elasticsearch documentation](https://www.elastic.co/guide/en/elasticsearch/reference/current/scalability.html). |
 | `Number of Elasticsearch replicas`                    | Each Elasticsearch shard can have a number of replicas. These are a complete copy of the shard, and can provide increased query performance or resilience against hardware failure. Increasing this value will greatly increase total disk space required by the index. |
 | `Limit namespaces and projects that can be indexed`   | Enabling this will allow you to select namespaces and projects to index. All other namespaces and projects will use database search instead. Please note that if you enable this option but do not select any namespaces or projects, none will be indexed. [Read more below](#limiting-namespaces-and-projects).
 | `Using AWS hosted Elasticsearch with IAM credentials` | Sign your Elasticsearch requests using [AWS IAM authorization](https://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_access-keys.html), [AWS EC2 Instance Profile Credentials](https://docs.aws.amazon.com/codedeploy/latest/userguide/getting-started-create-iam-instance-profile.html#getting-started-create-iam-instance-profile-cli), or [AWS ECS Tasks Credentials](https://docs.aws.amazon.com/AmazonECS/latest/userguide/task-iam-roles.html). Please refer to [Identity and Access Management in Amazon Elasticsearch Service](https://docs.aws.amazon.com/elasticsearch-service/latest/developerguide/es-ac.html) for details of AWS hosted Elasticsearch domain access policy configuration. |
@@ -476,7 +476,12 @@ For basic guidance on choosing a cluster configuration you may refer to [Elastic
 - `Heap size` should be set to no more than 50% of your physical RAM. Additionally, it shouldn't be set to more than the threshold for zero-based compressed oops. The exact threshold varies, but 26 GB is safe on most systems, but can also be as large as 30 GB on some systems. See [Heap size settings](https://www.elastic.co/guide/en/elasticsearch/reference/current/important-settings.html#heap-size-settings) and [Setting JVM options](https://www.elastic.co/guide/en/elasticsearch/reference/current/jvm-options.html) for more details.
 - Number of CPUs (CPU cores) per node usually corresponds to the `Number of Elasticsearch shards` setting described below.
 - A good guideline is to ensure you keep the number of shards per node below 20 per GB heap it has configured. A node with a 30GB heap should therefore have a maximum of 600 shards, but the further below this limit you can keep it the better. This will generally help the cluster stay in good health.
-- Small shards result in small segments, which increases overhead. Aim to keep the average shard size between at least a few GB and a few tens of GB. Another consideration is the number of documents, you should aim for this simple formula for the number of shards: `number of expected documents / 5M +1`.
+- Number of Elasticsearch shards:
+  - Small shards result in small segments, which increases overhead. Aim to keep the average shard size between at least a few GB and a few tens of GB.
+  - Another consideration is the number of documents. To determine the number of shards to use, sum the numbers in the **Admin Area > Dashboard > Statistics** pane (the number of documents to be indexed), divide by 5 million, and add 5. For example:
+    - If you have fewer than about 2,000,000 documents, use the default of 5 shards
+    - 10,000,000 documents: `10000000/5000000 + 5` = 7 shards
+    - 100,000,000 documents: `100000000/5000000 + 5` = 25 shards
 - `refresh_interval` is a per index setting. You may want to adjust that from default `1s` to a bigger value if you don't need data in real-time. This will change how soon you will see fresh results. If that's important for you, you should leave it as close as possible to the default value.
 - You might want to raise [`indices.memory.index_buffer_size`](https://www.elastic.co/guide/en/elasticsearch/reference/current/indexing-buffer.html) to 30% or 40% if you have a lot of heavy indexing operations.