Question about a sentence element

[learning54]

Hi Teachers,
Which sentence element is 'books' in the followinng question?
How many books are there on the shelf? Is it a subject?

Not long ago I've asked this question:
Which sentence element is 'tickets' in following question?
How many tickets is Caroline going to buy for the concert?
I've obtained these two answers:
1. Object.
2. 'Tickets' is the direct object of the verb 'buy'.
But this question already contains a subject 'Caroline'. How about the one with 'books'?


Thanks in advance