Question

how many后面为什么跟are there？

Answer 1

对there be句型中主语的数量如:some, five, only one等提问时,如果主语是可数名词,不管主语是单数还是复数一般都用复数形式提问,因为问话人不知道具体的数量是多少,而且many只能接可数名词复数形式,所以be一定要用are
即用How many+可数名词复数+are there+地点/时间状语?的句型结构.例如:

1. There is a book on the desk. (用how many改为特殊疑问句)
How many books are there on the desk?

2. There are seven days in a week. (对划线部分进行提问)
How many days are there in a week?

Answer 2

既然用how many ，那么就是用来计算可数名词的个数的，那么后面的be动词肯定要用复数了。