Control number of buckets created in an aggregation

Elastic Stack Elasticsearch
4

Here's the response of GET /my_index/_settings:

{
  "my_index" : {
    "settings" : {
      "index" : {
        "creation_date" : "1564578971559",
        "number_of_shards" : "1",
        "number_of_replicas" : "1",
        "uuid" : "KXlMMbZpT-yX8AaQK1FI7w",
        "version" : {
          "created" : "6080299",
          "upgraded" : "7030099"
        },
        "provided_name" : "my_index"
      }
    }
  }
}

i.e. my_index has only one shard.