Oracle won't complain if the number format is too long, so you can use a model that has enough digits to cope with the biggest number you can receive:
SQL> select to_number('12,345',
2 '999G999G999G999G999G999G999G999G999G999G999G999G999') from dual;
TO_NUMBER('12,345','999G999G999G999G999G999G999G999G999G999G999G999G999')
-------------------------------------------------------------------------
12345
SQL> select to_number('1,234,567',
2 '999G999G999G999G999G999G999G999G999G999G999G999G999') from dual;
TO_NUMBER('1,234,567','999G999G999G999G999G999G999G999G999G999G999G999G999')
----------------------------------------------------------------------------
1234567
SQL> select to_number('999,999,999,999,999,999,999,999,999,999,999,999,999',
2 '999G999G999G999G999G999G999G999G999G999G999G999G999') from dual;
TO_NUMBER('999,999,999,999,999,999,999,999,999,999,999,999,999','999G999G999G999
--------------------------------------------------------------------------------
1.0000E+39
I've used the G
group separator instead of a fixed comma to support globalisation, but the effect is the same.
The only caveat is that the source number has to have the right grouping so it matches the formatting exactly for the digits it does have:
SQL> select to_number('1,2345',
2 '999G999G999G999G999G999G999G999G999G999G999G999G999') from dual;
select to_number('1,2345',
*
ERROR at line 1:
ORA-01722: invalid number