Getting n slip number based on the number of times in a month a slip has been created
Use the Windowing Function ROW_NUMBER().

SELECT SLIPNO, EMPNO, DT, 
ROW_NUMBER() OVER (PARTITION BY EMPNO ORDER BY DT ASC) AS SLIPNUMBER
FROM SLIPS;


DB Fiddle Test to show an example.

Also note that you should not use DATE as a column name, as it is a reserved word.