The trigger below will only allow user A, B or C create up to 10 objects within the database. Once the 11th object is created then the database will throw up the raise application error.

create or replace trigger no_more_than_10
before create on database
declare
 l_cnt	number;
begin
  if (user in ('A','B','C'))
	then
		select count(*) into l_cnt
		from dba_objects
		where owner = USER
		and object_type = 'TABLE';

	if (l_cnt >= 10)
	then
	raise_application_Error (-20001, 'You can only create up to 10 objects');
	end if;
   
  end if;
end;