Sending multiple _count requests in one go

What version are you using? I can't reproduce that problem locally on 6.6.1.

DELETE /test
PUT /test/_doc/1
{
  "foo": "bar"
}
$ cat requests
{}
{ "query": {"match_all": {}}}

{ "query": {"match_all": {}}}

{ "query": {"match_all": {}}}

{ "query": {"match_all": {}}}
curl -H "Content-Type: application/x-ndjson" -XGET "http://127.0.0.1:9200/test/_msearch?pretty" --data-binary "@requests"; echo

Gives:

{
  "responses" : [
    {
      "took" : 6,
      "timed_out" : false,
      "_shards" : {
        "total" : 5,
        "successful" : 5,
        "skipped" : 0,
        "failed" : 0
      },
      "hits" : {
        "total" : 1,
        "max_score" : 1.0,
        "hits" : [
          {
            "_index" : "test",
            "_type" : "_doc",
            "_id" : "1",
            "_score" : 1.0,
            "_source" : {
              "foo" : "bar"
            }
          }
        ]
      },
      "status" : 200
    },
    {
      "took" : 4,
      "timed_out" : false,
      "_shards" : {
        "total" : 5,
        "successful" : 5,
        "skipped" : 0,
        "failed" : 0
      },
      "hits" : {
        "total" : 1,
        "max_score" : 1.0,
        "hits" : [
          {
            "_index" : "test",
            "_type" : "_doc",
            "_id" : "1",
            "_score" : 1.0,
            "_source" : {
              "foo" : "bar"
            }
          }
        ]
      },
      "status" : 200
    },
    {
      "took" : 5,
      "timed_out" : false,
      "_shards" : {
        "total" : 5,
        "successful" : 5,
        "skipped" : 0,
        "failed" : 0
      },
      "hits" : {
        "total" : 1,
        "max_score" : 1.0,
        "hits" : [
          {
            "_index" : "test",
            "_type" : "_doc",
            "_id" : "1",
            "_score" : 1.0,
            "_source" : {
              "foo" : "bar"
            }
          }
        ]
      },
      "status" : 200
    },
    {
      "took" : 4,
      "timed_out" : false,
      "_shards" : {
        "total" : 5,
        "successful" : 5,
        "skipped" : 0,
        "failed" : 0
      },
      "hits" : {
        "total" : 1,
        "max_score" : 1.0,
        "hits" : [
          {
            "_index" : "test",
            "_type" : "_doc",
            "_id" : "1",
            "_score" : 1.0,
            "_source" : {
              "foo" : "bar"
            }
          }
        ]
      },
      "status" : 200
    }
  ]
}