问句是：How much does Mary weigh ?
意思是：玛丽重多少?
How much 后面跟不可数名词,或者不跟 名词.
How many 后面必须跟可数名词复数形式.
例如：
How much do you like watching TV? (how much 后面不跟名词)
How much water do you have in the bottle ?你瓶子里有多少水?（how much +不可数名词）
How many students are there in your school?你们学校有多少学生?
希望能帮到你,祝你学习进步!