UPDATE 테이블 a
   SET 빈컬럼 = (SELECT rn
                   FROM (SELECT ROWID rid
                              , ROW_NUMBER() OVER(ORDER BY 특정컬럼) rn
                           FROM 테이블
                         )
                  WHERE rid = a.ROWID
                 )
 WHERE ROWID IN (SELECT rid
                   FROM (SELECT ROWID rid
                              , ROW_NUMBER() OVER(ORDER BY 특정컬럼) rn
                           FROM 테이블
                         )
                  WHERE rn <= 500
                 )