SQL> CREATE OR REPLACE
  2    TYPE NUMBER_0_20
  3      AS OBJECT (
  4                 N NUMBER,
  5                 CONSTRUCTOR FUNCTION NUMBER_0_20(
  6                                                  SELF IN OUT NOCOPY NUMBER_0_20,
  7                                                  N NUMBER
  8                                                 )
  9                   RETURN SELF AS RESULT
 10                )
 11  /

Type created.

SQL> CREATE OR REPLACE
  2    TYPE BODY NUMBER_0_20
  3      AS
  4        CONSTRUCTOR FUNCTION NUMBER_0_20(
  5                                         SELF IN OUT NOCOPY NUMBER_0_20,
  6                                         N NUMBER
  7                                        )
  8                   RETURN SELF AS RESULT
  9          IS
 10          BEGIN
 11              IF NVL(N,1) BETWEEN 0 AND 20
 12                THEN
 13                  SELF.N := N;
 14                  RETURN;
 15              END IF;
 16              RAISE_APPLICATION_ERROR(
 17                                      -20500,
 18                                      'Value must be between 0 and 20.'
 19                                     );
 20        END;
 21  END;
 22  /

Type body created.

SQL> set numwidth 20
SQL> SELECT  NUMBER_0_20(3.141592653589793238)
  2    FROM  DUAL
  3  /

NUMBER_0_20(3.141592653589793238)(N)
-----------------------------------------------------------------------------------------------------------------
NUMBER_0_20(3.141592653589793238)

SQL> SELECT  NUMBER_0_20(55)
  2    FROM  DUAL
  3  /
SELECT  NUMBER_0_20(55)
        *
ERROR at line 1:
ORA-20500: Value must be between 0 and 20.
ORA-06512: at "SCOTT.NUMBER_0_20", line 15


SQL>