SQL> create or replace trigger test_trigger
  2  after create on schema
  3  begin
  4    <<bk>>
  5    declare
  6      cnt pls_integer := 0;
  7    begin
  8      if sys.dictionary_obj_type = 'TABLE' then
  9        select
 10          count(*)
 11        into
 12          bk.cnt
 13        from
 14          user_tab_columns t1
 15        where
 16          t1.table_name = sys.dictionary_obj_name;
 17  --
 18        if bk.cnt > 5 then
 19          raise_application_error(-20001, 'too many columns');
 20        end if;
 21      end if;
 22    end;
 23  end;
 24  /

Trigger created.

SQL> show errors;
No errors.
SQL> 
SQL> create table t1(n number);

Table created.


SQL> create table t2(n1 number, n2 number, n3 number, n4 number, n5 number, n6 number);
create table t2(n1 number, n2 number, n3 number, n4 number, n5 number, n6 number)
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-20001: too many columns
ORA-06512: at line 17