declare @BenchTest as table(ID  int Identity(1,1), PartNumber  int
, PartName varchar(10), TestDate datetime default getdate());

insert into @BenchTest(PartNumber, PartName)
values(1,'A'),(2,'B'),(3,'C'),(4,'D'),(1,'A'),(2,'B'),(3,'C')
,(4,'D'),(1,'A'),(2,'B'),(3,'C'),(4,'D'),(1,'A'),(2,'B')

select ID
, PartNumber
, PartName
, ROW_NUMBER()Over(Partition by PartNumber Order by (Select Null)) CycleNum
, count(*)Over(Partition by PartNumber) TotalCycle
, TestDate
from @BenchTest
order by PartNumber

DECLARE @BenchTest TABLE 
    (ID INT IDENTITY
    ,PartNumber INT
    ,PartName VARCHAR(10)
    ,TestDate DATETIME);

INSERT INTO @BenchTest(PartNumber, PartName, TestDate)
VALUES(1,'A','2012-01-01 11:00'),(2,'B','2012-01-01 12:00')
    ,(3,'C','2012-01-01 13:00'),(4,'D','2012-01-01 14:00')
    ,(1,'A','2012-01-02 12:00'),(2,'B','2012-01-02 13:00')
    ,(3,'C','2012-01-02 14:00'),(4,'D','2012-01-01 15:00')
    ,(1,'A','2012-01-03 12:00'),(2,'B','2012-01-03 13:00')
    ,(3,'C','2012-01-03 15:00'),(4,'D','2012-01-01 15:00')
    ,(1,'A','2012-01-04 12:00'),(2,'B','2012-01-04 12:00')

SELECT ID
    ,PartNumber
    ,PartName
    ,CycleNum=ROW_NUMBER() OVER (PARTITION BY PartNumber ORDER BY ID) 
    ,TotalCycle=COUNT(*)Over(Partition by PartNumber) 
    ,TestDate
FROM @BenchTest
ORDER BY PartNumber