account_ids = [
    "111111111111",
    "222222222222"
  ]

data "aws_iam_policy_document" "sf-automation-switch-role" {
  statement {
    effect = "Allow"
    actions = ["sts:AssumeRole"]
    # resources = ["${data.template_file.account_ids.*.rendered}"]
      }
}

data "template_file" "account_ids" {
  count = "${length(var.account_ids)}"
  # need to double-escape template variables
  # We need this list to have " ", therefore it is escaped.
  template = ""arn:aws::$${account_id}:role/$${assumed-automation-role}""
  vars {
    account_id = "${var.account_ids[count.index]}"
    assumed-automation-role = "${var.assumed-automation-role}"
  }
}

""arn:aws::111111111:role/sf-automation-id""