Oracle 21c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (from_date, until_date) AS
SELECT DATE '2020-03-17', DATE '2020-05-18' FROM DUAL UNION ALL
SELECT DATE '2020-05-18', DATE '2020-06-08' FROM DUAL UNION ALL
SELECT DATE '2020-12-21', DATE '2021-03-01' FROM DUAL UNION ALL
SELECT DATE '2021-03-01', DATE '2021-03-11' FROM DUAL UNION ALL
SELECT DATE '2021-10-19', DATE '2021-10-22' FROM DUAL UNION ALL
SELECT DATE '2022-01-10', DATE '2022-01-14' FROM DUAL UNION ALL
SELECT DATE '2022-01-14', NULL FROM DUAL;
7 rows affected

SELECT from_date,
       until_date,
       COALESCE(until_date, TRUNC(SYSDATE)) - from_date
         + CASE
           WHEN LAG(until_date) OVER (ORDER BY FROM_DATE) = from_date
           THEN 0
           ELSE 1
           END
         AS difference
FROM   table_name;


FROM_DATE
UNTIL_DATE
DIFFERENCE


2020-03-17 00:00:00
2020-05-18 00:00:00
63

2020-05-18 00:00:00
2020-06-08 00:00:00
21

2020-12-21 00:00:00
2021-03-01 00:00:00
71

2021-03-01 00:00:00
2021-03-11 00:00:00
10

2021-10-19 00:00:00
2021-10-22 00:00:00
4

2022-01-10 00:00:00
2022-01-14 00:00:00
5

2022-01-14 00:00:00
null
154


BEGIN
  FOR r IN (
    SELECT from_date,
           until_date,
           COALESCE(until_date, TRUNC(SYSDATE)) - from_date
             + CASE
               WHEN LAG(until_date) OVER (ORDER BY FROM_DATE) = from_date
               THEN 0
               ELSE 1
               END
             AS difference
    FROM   table_name
  ) LOOP
    DBMS_OUTPUT.PUT_LINE( r.from_date || ', ' || r.until_date || ', ' || r.difference );
  END LOOP;
END;
/
1 rows affected

dbms_output:
2020-03-17 00:00:00, 2020-05-18 00:00:00, 63
2020-05-18 00:00:00, 2020-06-08 00:00:00, 21
2020-12-21 00:00:00, 2021-03-01 00:00:00, 71
2021-03-01 00:00:00, 2021-03-11 00:00:00, 10
2021-10-19 00:00:00, 2021-10-22 00:00:00, 4
2022-01-10 00:00:00, 2022-01-14 00:00:00, 5
2022-01-14 00:00:00, , 154

DECLARE
  v_until_date DATE;
  v_diff       NUMBER;
BEGIN
  FOR r IN (
    SELECT from_date,
           until_date
    FROM   table_name
    ORDER BY from_date
  ) LOOP
    v_diff := COALESCE(r.until_date, TRUNC(SYSDATE)) - r.from_date
            + CASE WHEN v_until_date = r.from_date THEN 0 ELSE 1 END;
    DBMS_OUTPUT.PUT_LINE(
      r.from_date
      || ', ' || r.until_date
      || ', ' || v_diff
    );
    v_until_date := r.until_date;
  END LOOP;
END;
/
1 rows affected

dbms_output:
2020-03-17 00:00:00, 2020-05-18 00:00:00, 63
2020-05-18 00:00:00, 2020-06-08 00:00:00, 21
2020-12-21 00:00:00, 2021-03-01 00:00:00, 71
2021-03-01 00:00:00, 2021-03-11 00:00:00, 10
2021-10-19 00:00:00, 2021-10-22 00:00:00, 4
2022-01-10 00:00:00, 2022-01-14 00:00:00, 5
2022-01-14 00:00:00, , 154

FROM_DATE	UNTIL_DATE	DIFFERENCE
2020-03-17 00:00:00	2020-05-18 00:00:00	63
2020-05-18 00:00:00	2020-06-08 00:00:00	21
2020-12-21 00:00:00	2021-03-01 00:00:00	71
2021-03-01 00:00:00	2021-03-11 00:00:00	10
2021-10-19 00:00:00	2021-10-22 00:00:00	4
2022-01-10 00:00:00	2022-01-14 00:00:00	5
2022-01-14 00:00:00	null	154