Here is another, slightly different solution:

SELECT x.character, COUNT(emp.empno)
FROM emp,
----(SELECT CHR(ROWNUM - 1 + ASCII('A')) character
-------FROM all_objects
-------WHERE ROWNUM <= ASCII('Z') - ASCII('A') + 1
-----) x
WHERE x.character = SUBSTR(emp.ename(+),1,1)
GROUP BY x.character;