how often问的是“多经常”?,也就是动作发生的频率,多久一/两/三/n次
如：
How often do you go to see your parents?
Twice a week.
而
she has been to america twice
提问：twice两次,没有频率的意思,而是发生的次数,所以用how many times
How many times have you seen the movie?
Once一次
Twice两次
Three times.三次
Four times.四次