create or replace function split_by_seq(c varchar2, m number)
  return varchar2 as
  st number := 1;
  ed number;
  l  number;
  n  number := m;

begin
  if c is null or m is null then
    return 0;
  end if;

  loop
    ed := instr(c, ',', st);
    if (ed = 0 and n - 1 <> 0) then
      return 'Input errors!';
    end if;
    n := n - 1;
    if n = 0 then
      exit;
    else
      st := ed + 1;
    end if;
 
  end loop;
  if ed = 0 then
    return substr(c, st);
  end if;
  l := ed - st;
  return substr(c, st, l);

  return 0;
end;

-------------------------------------------------------------------------------------------------------------

SQL> select split_by_seq('123,456,789,987,654,321,012,120',4) from dual;
 
SPLIT_BY_SEQ('123,456,789,987,
--------------------------------------------------------------------------------
987
 
SQL>