GET /yourindex/_search?search_type=count
{
  "aggs": {
    "yourGroup": {
      "terms": {
        "field": "field1",
        "size": 10
      },
      "aggs": {
        "theMax": {
          "max": {
            "field": "field2"
          }
        }
      }
    }
  }
}