Oracle 21c

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CREATE TABLE employees (Employee_ID, StartDt, EndDt ) AS
SELECT 123, DATE '2022-02-01', NULL FROM DUAL UNION ALL
SELECT 234, DATE '2022-01-01', DATE '2022-02-28' FROM DUAL UNION ALL
SELECT 456, DATE '2021-12-01', DATE '2022-02-28' FROM DUAL;
3 rows affected

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
WITH calendar (month) AS (
  SELECT ADD_MONTHS(min_startdt, LEVEL - 1)
  FROM   (
    SELECT MIN(startdt) AS min_startdt,
           MAX(COALESCE(enddt, SYSDATE)) AS max_enddt
    FROM   employees
  )
  CONNECT BY LEVEL - 1 <= MONTHS_BETWEEN(max_enddt, min_startdt)
)
SELECT c.month,
       COUNT(e.employee_id)
FROM   calendar c
       LEFT OUTER JOIN employees e
       ON (e.startdt <= c.month AND (c.month <= e.enddt OR e.enddt IS NULL))
GROUP BY
       c.month


MONTH
COUNT(E.EMPLOYEE_ID)


2021-12-01 00:00:00
1

2022-01-01 00:00:00
2

2022-02-01 00:00:00
3

2022-03-01 00:00:00
1

2022-04-01 00:00:00
1

2022-05-01 00:00:00
1

2022-06-01 00:00:00
1

2022-07-01 00:00:00
1

2022-08-01 00:00:00
1

2022-09-01 00:00:00
1

MONTH	COUNT(E.EMPLOYEE_ID)
2021-12-01 00:00:00	1
2022-01-01 00:00:00	2
2022-02-01 00:00:00	3
2022-03-01 00:00:00	1
2022-04-01 00:00:00	1
2022-05-01 00:00:00	1
2022-06-01 00:00:00	1
2022-07-01 00:00:00	1
2022-08-01 00:00:00	1
2022-09-01 00:00:00	1