select rn||'月份' ,count(dat)||'（个人）' from 
(select rownum rn from dual connect by rownum <=12) t1
,
(select substr(to_char(登记时间,'YYYYMMDD'),5,2) dat from a)t2
where t1.rn=t2.dat(+) group by rn order by rn

------解决方案--------------------
SQL code

WITH T1 AS(
    SELECT ADD_MONTHS(DATE'2010-12-01',LEVEL) AS t_date
    FROM DUAL CONNECT BY LEVEL <= 12
  ),T2 AS(SELECT '张三' 姓名,date'2011-12-23' 时间 , '王武' 登记人 FROM DUAL UNION ALL
          SELECT '李四' 姓名,date'2011-12-25' 时间 , '刘敏' 登记人 FROM DUAL UNION ALL
          SELECT '周松' 姓名,date'2011-11-24' 时间 , '王武' 登记人 FROM DUAL UNION ALL
          SELECT '赵思' 姓名,date'2011-10-22' 时间 , '王武' 登记人 FROM DUAL UNION ALL
          SELECT '赵思' 姓名,date'2011-10-01' 时间 , '王武' 登记人 FROM DUAL UNION ALL
          SELECT '钱斯' 姓名,date'2011-04-06' 时间 , '张松' 登记人 FROM DUAL UNION ALL
          SELECT '周松' 姓名,date'2011-03-24' 时间 , '王武' 登记人 FROM DUAL UNION ALL
          SELECT '钱斯' 姓名,date'2011-01-06' 时间 , '张松' 登记人 FROM DUAL
  )SELECT TO_CHAR(T1.t_date,'YYYY-MM') 月份,NVL(count(T2.时间),0)||'(个人)' 总人数
     FROM T1 LEFT JOIN T2 ON TO_CHAR(T1.t_date,'YYYY-MM') = TO_CHAR(T2.时间,'YYYY-MM')
    GROUP BY TO_CHAR(T1.t_date,'YYYY-MM')
    ORDER BY TO_CHAR(T1.t_date,'YYYY-MM')

-------------------------------------------------
          月份  总人数
1    2011-01    1(个人)
2    2011-02    0(个人)
3    2011-03    1(个人)
4    2011-04    1(个人)
5    2011-05    0(个人)
6    2011-06    0(个人)
7    2011-07    0(个人)
8    2011-08    0(个人)
9    2011-09    0(个人)
10    2011-10    2(个人)
11    2011-11    1(个人)
12    2011-12    2(个人)