create trigger x after insert on t1
for each row 
    insert into t2 values (new.id);

------解决方案--------------------
楼主的触发器动作错了吧.应该是insert.不然怎么会取到new.id;
或则楼主想表达的是这个意思:

delimiter $$
create trigger trig after update on t1
for each row
begin
 update t2 set num=num+1 where id=old.id;   #(如何获取当前t1更新行ID？) ;//在t2表对应的ID更新数据
end$$
delimiter ;

------解决方案--------------------
select last_insert_id();