select count(1),C2 from tab1 group by C2;

COUNT(1)	   C2
11	   	   -23
6	   	   -1
4613	  	   2
3		   3
21993451	   4
45		   5
10		   13
17		   99
403295	           null

SQL> create table t 
  2  as 
  3  select rownum c1,
  4         case when rownum < 2 then 1
  5              when rownum < 10 then 2
  6              when rownum < 50 then 3
  7              when rownum < 1000 then 10
  8              end c2
  9  from all_objects
 10  where rownum < 3000
 11  /

Table created.

SQL> select count(*),C2 from t group by C2 order by c2;
  COUNT(*)         C2
---------- ----------
         1          1
         8          2
        40          3
       950         10
      2000

5 rows selected.

SQL> exec dbms_stats.gather_table_stats(user,'T',METHOD_OPT=>'FOR COLUMNS C1 SIZE 1 COLUMNS C2 SIZE 254');

PL/SQL procedure successfully completed.

SQL> select column_name, num_buckets from user_tab_col_statistics where table_name='T';
COLUMN_NAME                    NUM_BUCKETS
------------------------------ -----------
C1                                       1
C2                                       4

2 rows selected.