DECLARE @T_Table TABLE (ID INT, RepID INT, Rep_No VARCHAR(10))

INSERT INTO @T_Table 
			SELECT 1, 0, Null
UNION ALL	SELECT 2, 0, Null
UNION ALL	SELECT 3, 3, Null
UNION ALL	SELECT 4, 3, Null
UNION ALL	SELECT 4, 4, Null
UNION ALL	SELECT 5, 4, Null
UNION ALL	SELECT 6, 6, Null
UNION ALL	SELECT 7, 4, Null

SELECT		*,
			CONVERT(VARCHAR(10), ROW_NUMBER() OVER (PARTITION BY RepID ORDER BY Id)) 
			+ ' of ' +
			CONVERT(VARCHAR(10), COUNT(ID) OVER (PARTITION BY RepID)) AS 'NewRep_No'
FROM		@T_Table 
ORDER BY	ID

"There is only one difference between a dream and an aim.
A dream requires soundless sleep to see,
whereas an aim requires sleepless efforts to achieve..!!"