Remove shards per gb of heap guidance (#86223)

This guidance does not apply any longer.
The overhead per shard has been significantly reduced in recent versions
and removed rule of thumb will be too pessimistic in many if not
most cases and might be too optimistic in other specific ones.

=> Replace guidance with rule of thumb per field count on data nodes and
rule of thumb by index count (which is far more relevant nowadays than
shards) for master nodes.

relates #77466

Co-authored-by: David Turner <david.turner@elastic.co>
Co-authored-by: Henning Andersen <33268011+henningandersen@users.noreply.github.com>

docs/reference/how-to/size-your-shards.asciidoc

@@ -55,7 +55,7 @@ thread pool>>. This can result in low throughput and slow search speeds.
 
 [discrete]
 [[each-shard-has-overhead]]
-==== Each index and shard has overhead
+==== Each index, shard and field has overhead
 
 Every index and every shard requires some memory and CPU resources. In most
 cases, a small set of large shards uses fewer resources than many small shards.
@@ -175,17 +175,17 @@ index                                 prirep shard store
 
 [discrete]
 [[shard-count-recommendation]]
-==== Aim for 20 shards or fewer per GB of heap memory
+==== Aim for 3000 indices or fewer per GB of heap memory on each master node
 
-The number of shards a data node can hold is proportional to the node's heap
-memory. For example, a node with 30GB of heap memory should have at most 600
-shards. The further below this limit you can keep your nodes, the better. If
-you find your nodes exceeding more than 20 shards per GB, consider adding
-another node.
+The number of indices a master node can manage is proportional to its heap
+size. The exact amount of heap memory needed for each index depends on various
+factors such as the size of the mapping and the number of shards per index.
 
-Some system indices for {enterprise-search-ref}/index.html[Enterprise Search]
-are nearly empty and rarely used. Due to their low overhead, you shouldn't
-count shards for these indices toward a node's shard limit.
+As a general rule of thumb, you should aim for 3000 indices or fewer per GB of
+heap on master nodes. For example, if your cluster contains 12000 indices then
+each dedicated master node should have at least 4GB of heap. For non-dedicated
+master nodes, the same rule holds and should be added to the heap requirements
+of the other roles of each node.
 
 To check the configured size of each node's heap, use the <<cat-nodes,cat nodes
 API>>.
@@ -205,6 +205,23 @@ GET _cat/shards?v=true
 ----
 // TEST[setup:my_index]
 
+[discrete]
+[[field-count-recommendation]]
+==== Allow 1kB of heap per field per index on data nodes, plus overheads
+
+The exact resource usage of each mapped field depends on its type, but a rule
+of thumb is to allow for approximately 1kB of heap overhead per mapped field
+per index held by each data node. You must also allow enough heap for {es}'s
+baseline usage as well as your workload such as indexing, searches and
+aggregations. 0.5GB of extra heap will suffice for many reasonable workloads,
+and you may need even less if your workload is very light while heavy workloads
+may require more.
+
+For example, if a data node holds shards from 1000 indices, each containing
+4000 mapped fields, then you should allow approximately 1000 × 4000 × 1kB = 4GB
+of heap for the fields and another 0.5GB of heap for its workload and other
+overheads, and therefore this node will need a heap size of at least 4.5GB.
+
 [discrete]
 [[avoid-node-hotspots]]
 ==== Avoid node hotspots