select deptno, count(*) no_emp from emp group by deptno;

    this will give you like

deptno          no_emp

10                  4

20                   5

...........

now to put this result in a single row use:

select count(decode(deptno,10, deptno, null)) No_empof_10dept,

count(decode(deptno,20,deptno,null)) no_empof_20dept,

count(decode(deptno,10,null, 20 , null, deptno)) otherthen_10_20

from emp;