DEMO: 

 var listTest = new List<RECORD_DETAIL>();
 listTest.Add(new RECORD_DETAIL { SYS_ID = 1, SYS_NO = "A" });
 listTest.Add(new RECORD_DETAIL { SYS_ID = 1, SYS_NO = "AA" });
 listTest.Add(new RECORD_DETAIL { SYS_ID = 2, SYS_NO = "B" });

 var modelsGroup = listTest.GroupBy(x => x.SYS_ID).ToList();
 IGrouping<int, RECORD_DETAIL> models = modelsGroup[0];
 int groupCount = models.Count<RECORD_DETAIL>();