SELECT COUNT(*) 
FROM (SELECT xyz
      FROM abc
      GROUP BY xyz
      HAVING COUNT(*) = 1
     );

SELECT COUNT(*) 
FROM abc
WHERE xyz NOT IN (SELECT xyz
                  FROM abc
                  GROUP BY xyz
                  HAVING COUNT(*) > 1
                 );

orclz> create table abc(xyz number);

Table created.

orclz> insert into abc(select rownum from dual connect by level < 501);

500 rows created.

orclz> insert into abc(select rownum from dual connect by level < 11);

10 rows created.

orclz> commit;

Commit complete.

orclz> set autot trace exp
orclz> SELECT COUNT(*)
  2  FROM (SELECT xyz
  3        FROM abc
  4        GROUP BY xyz
  5        HAVING COUNT(*) = 1
  6       );

Execution Plan
----------------------------------------------------------
Plan hash value: 1018226419

------------------------------------------------------------------------------
| Id  | Operation             | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |      |     1 |       |     4  (25)| 00:00:01 |
|   1 |  SORT AGGREGATE       |      |     1 |       |            |          |
|   2 |   VIEW                |      |     5 |       |     4  (25)| 00:00:01 |
|*  3 |    FILTER             |      |       |       |            |          |
|   4 |     HASH GROUP BY     |      |     5 |    20 |     4  (25)| 00:00:01 |
|   5 |      TABLE ACCESS FULL| ABC  |   510 |  2040 |     3   (0)| 00:00:01 |
------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter(COUNT(*)=1)

orclz> SELECT COUNT(*)
  2  FROM abc
  3  WHERE xyz NOT IN (SELECT xyz
  4                    FROM abc
  5                    GROUP BY xyz
  6                    HAVING COUNT(*) > 1
  7                   );

Execution Plan
----------------------------------------------------------
Plan hash value: 3850865817

-----------------------------------------------------------------------------------
| Id  | Operation              | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |          |     1 |    17 |     7  (15)| 00:00:01 |
|   1 |  SORT AGGREGATE        |          |     1 |    17 |            |          |
|*  2 |   HASH JOIN ANTI NA    |          |   485 |  8245 |     7  (15)| 00:00:01 |
|   3 |    TABLE ACCESS FULL   | ABC      |   510 |  2040 |     3   (0)| 00:00:01 |
|   4 |    VIEW                | VW_NSO_1 |    25 |   325 |     4  (25)| 00:00:01 |
|*  5 |     FILTER             |          |       |       |            |          |
|   6 |      HASH GROUP BY     |          |    25 |   100 |     4  (25)| 00:00:01 |
|   7 |       TABLE ACCESS FULL| ABC      |   510 |  2040 |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("XYZ"="XYZ")
   5 - filter(COUNT(*)>1)

select count(a) from t1 where a not in(
select a from t1 group by a having count(a)>1);