How to get the number of jsons under the field?

There is a json like this. It's not a list of jsons inside but rather a separate json on unique identifier.

{
  "intro": {
    ...
  },
  "data": {
    "1001": {
      "id": 9,
      "value": 115
    },
    ...
    "1009": {
      "id": 7,
      "value": 410
    }
  }
}

It's easy to get the value of data but how to get a name of a key for the last json in data (i.e. it is 1009) ?

for get a name of a key for the last json in data:

import 'dart:convert';

void main() {
  final jsonString = '''
  {
    "intro": {},
    "data": {
      "1001": {
        "id": 9,
        "value": 115
      },
      "1002": {
        "id": 7,
        "value": 410
      },
      "1009": {
        "id": 7,
        "value": 410
      }
    }
  }
  ''';

  final jsonMap = json.decode(jsonString);
  final lastKey = jsonMap['data'].keys.last;
  print(lastKey); // getting lastkey value is "1009"
}

for get the number of jsons under the field?

import 'dart:convert';

void main() {
  final jsonString = '''
  {
    "intro": {},
    "data": {
      "1001": {
        "id": 9,
        "value": 115
      },
      "1002": {
        "id": 7,
        "value": 410
      },
      "1009": {
        "id": 7,
        "value": 410
      }
    }
  }
  ''';

  final jsonMap = json.decode(jsonString);
  final dataMap = jsonMap['data'];
  final numberOfJsons = dataMap.length;
  print(numberOfJsons); // getting number of json's length is 3
}