subnet.mainとかaws_security_groupなどなどは別途設定済みとして。

#--------------------------------------------------------------
# Ec2 Instances
#--------------------------------------------------------------
resource "aws_instance" "elasticsearch" {
  instance_type = "${var.elasticsearch_instance_type}"
  ami = "${var.default_ami}"
  subnet_id = "${element(aws_subnet.main.*.id, count.index%length(aws_subnet.main.*.id))}"
  vpc_security_group_ids = [ "${aws_security_group.output_all.id}", "${aws_security_group.elasticsearch.id}" ]
  key_name = "${aws_key_pair.developer.id}"
  iam_instance_profile = "${aws_iam_instance_profile.elasticsearch.id}"
  tags {
    Name = "${format("%s-elasticsearch%02d", var.environment, count.index+1)}"
    role = "elasticsearch"
    environment = "${var.environment}"
  }

  count = "${var.elasticsearch_server_count}"
}

#--------------------------------------------------------------
# EBS
#--------------------------------------------------------------
resource "aws_ebs_volume" "elasticsearch-data" {
  availability_zone = "${element(aws_subnet.main.*.availability_zone, count.index)}"
  size = "${var.elasticsearch_volume_size}"
  type = "gp2"
  tags {
    Name = "${format("elasticsearch%02d-data", count.index+1)}"
    environment = "${var.environment}"
  }
  count = "${var.elasticsearch_ebs_count}"
}

#--------------------------------------------------------------
# EBS attachment
#--------------------------------------------------------------
resource "aws_volume_attachment" "attach_elasticsearch-data" {
  device_name = "/dev/sdb"
  volume_id = "${element(aws_ebs_volume.elasticsearch-data.*.id, count.index)}"
  instance_id = "${element(aws_instance.elasticsearch.*.id, count.index)}"
  count = "${var.elasticsearch_server_count}"
}

これでelasticsearch_server_countだけサーバが立ち上がった上でEBSがattachされます。

さらに

terraform destory -target 'aws_instance.elasticsearch'

とした場合、インスタンスだけ消えてEBSは残るので、次回インスタンスを立ち上げたときには同じEBSが割り当てられます。

問題はterraform destoryしたときにEBSがマウント済みだと当然エラーになるので落とすときはumountをする何かが必要になります。
いまのところ作成時にのみprovisionerが実行されるだけなのでterraformでは完結しません。