ALTER TABLE cf_Stamp ADD lastMarkNum INT;
ALTER TABLE cf_StampMark ADD num INT;

--

UPDATE s
SET
    s.lastMarkNum = (
        SELECT COUNT(*)
        FROM cf_StampMark sm 
        WHERE sm.stampId = s.id
    )
FROM cf_Stamp s;

-- 

CREATE TABLE #tmpMarkNum (
  markId int,
  num int
);

DECLARE @idx INT;
SET @idx = 0;

INSERT INTO #tmpMarkNum (markId, num)
  SELECT
    id,
    ROW_NUMBER() OVER (PARTITION BY stampId ORDER BY id)
  FROM cf_StampMark
  ORDER BY stampId, id;

UPDATE m
SET m.num = mn.num
FROM cf_StampMark m
INNER JOIN #tmpMarkNum mn ON mn.markId = m.id;

DROP TABLE #tmpMarkNum;