ALTER TRIGGER Tr_user_count
ON dbo.[User] 
for UPDATE
AS
BEGIN
INSERT INTO [Count] (UserNum) 
select UserNum from [User]
end

--Try

------解决方案--------------------
ALTER TRIGGER Tr_user_count
ON dbo.[User] for UPDATE
AS
BEGIN
INSERT INTO Count (UserNum) (select UserNum from [User])
end

或者

INSERT INTO Count (UserNum) (select UserNum from inserted)

------解决方案--------------------
COUNT是SQL的关键字
你把COUNT加上[]看看

------解决方案--------------------
instead of 触发器是不执行触发触发器的操作，转而执行触发器内部定义的操作

for触发器实现执行触发触发器的操作，在执行触发器内部定义的操作，相当于after触发器

------解决方案--------------------
instead of UPDATE--

改为
for UPDATE

or

after update