WITH t AS
(
SELECT 1 a, 'a' b FROM dual
UNION ALL SELECT 1, 'b' FROM dual
UNION ALL SELECT 2, 'c' FROM dual
UNION ALL SELECT 2, 'd' FROM dual
UNION ALL SELECT 2, 'e' FROM dual
UNION ALL SELECT 2, 'f' FROM dual
UNION ALL SELECT 3, 'g' FROM dual
UNION ALL SELECT 3, 'h' FROM dual
UNION ALL SELECT 3, 'i' FROM dual
UNION ALL SELECT 3, 'j' FROM dual
UNION ALL SELECT 3, 'k' FROM dual
UNION ALL SELECT 3, 'l' FROM dual
)
SELECT CONNECT_BY_ROOT(a) st_dt
     , a ed_dt
     , LENGTH(REPLACE(SYS_CONNECT_BY_PATH(b, ','), ',')) cnt
  FROM (SELECT a
             , LISTAGG(b) WITHIN GROUP(ORDER BY b) b
             , ROW_NUMBER() OVER(ORDER BY a) rn
          FROM t
         GROUP BY a
        )
 CONNECT BY PRIOR rn + 1 = rn
 ORDER BY ed_dt, st_dt
;