How often:是对频率的提问
例如：how often does he purchase
他多久采购一次?
how many times :就是对次数进行提问：
如：how many times did he purchase in this year?
今年他采购了几次?
How many times就次数提问
how often 就频率提问
how often意为“多久……次、是否经常”,用来提问在某一特定的时间进行某个动作的次数,答语通常是always,usually,often,sometimes,once/twice a day/month等.
—How often do you get to school very early?你多久早到校一次?
—Usually.通常.
—How often do you see a film?你多久看一次电影?
—Once a month.每月一次.
其他知识点
how long,how often,how soon三者都可以译成“多久”.how long用来提问“多长时间”,对“一段时间的长短”提问时用,与延续性动词连用；how often用来提问某动作或状态发生的频率,对“一段时间内发生了几次活动”（always,usually,often,never或twice a day等表示频度的词或短语）提问时用；how soon用来提问某人要“多快”能做好某事,或某事要“多快”能完成,对“过多少时间（就能……）”（in短语）提问时用,常用于将来时.而how far意思是“多远”,对距离提问时用.例如：
How long did you stay there?你在那儿呆多久?
How often do his parents let him watch TV?他父母多久让他看一次电视?
How soon will her husband come?她丈夫多久会回来?
