TO_NUMBER('1.99999999999999',9999999.99999999999999999999)
----------------------------------------------------------
2

SQL> set numwidth 10
SQL> SELECT to_number('1.99999999999999',9999999.99999999999999999999) from dual;

TO_NUMBER('1.99999999999999',9999999.99999999999999999999)
----------------------------------------------------------
2

SQL> SELECT to_number('1.99999999',9999999.99999999999999999999) from dual;

TO_NUMBER('1.99999999',9999999.99999999999999999999)
----------------------------------------------------
1.99999999

SQL> SELECT to_number('1.999999999',9999999.99999999999999999999) from dual;

TO_NUMBER('1.999999999',9999999.99999999999999999999)
-----------------------------------------------------
2

SQL> set numwidth 15
SQL> SELECT to_number('1.999999999',9999999.99999999999999999999) from dual;

TO_NUMBER('1.999999999',9999999.99999999999999999999)
-----------------------------------------------------
1.999999999

SQL> SELECT to_number('1.9999999999999',9999999.99999999999999999999) from dual;

TO_NUMBER('1.9999999999999',9999999.99999999999999999999)
---------------------------------------------------------
1.9999999999999

SQL> SELECT to_number('1.99999999999999',9999999.99999999999999999999) from dual;

TO_NUMBER('1.99999999999999',9999999.99999999999999999999)
----------------------------------------------------------
2

SQL> spool off