SQL> Select count(*) nbr
  2       , deptno
  3    From emp
  4   Group by deptno;

       NBR     DEPTNO
---------- ----------
         3         10
         5         20
         6         30

SQL> select deptno
  2       , max(decode(my_seq,1,rtrim(ename)))||
  3         max(decode(my_seq,2,nvl2(ename,', '|| rtrim(ename),null)))||
  4         max(decode(my_seq,3,nvl2(ename,', '|| rtrim(ename),null)))||
  5         max(decode(my_seq,4,nvl2(ename,', '|| rtrim(ename),null)))||
  6         max(decode(my_seq,5,nvl2(ename,', '|| rtrim(ename),null)))||
  7         max(decode(my_seq,6,nvl2(ename,', '|| rtrim(ename),null))) emp_list
  8    from ( select deptno
  9         , ename
 10         , row_number() over (partition by deptno order by ename) my_seq
 11          from emp
 12          )
 13    group by deptno
 14    order by 1
 15  /

    DEPTNO EMP_LIST
---------- ----------------------------------------------------------------------
        10 CLARK, KING, MILLER
        20 ADAMS, FORD, JONES, SCOTT, SMITH
        30 ALLEN, BLAKE, JAMES, MARTIN, TURNER, WARD