It means "there are six books".
"hay tres libros sobre el escritorio"
It means there are books in the library.
The phrase "en la mesa hay mucho libros" is incorrect. It should be "en la mesa hay muchos libros" to match the plural form of "libros."
cuantas habitaciones cuantos cuartos
"Hay" means, "there is" or "there are."
It doesn't make sense as written. 100 = cien; 'libro' = '(a) book', or 'I deliver'; 'la biblioteca' = 'the library' If 'libro' were 'libros', '100 libros' = '100 books', but no specific connection is suggested with 'la biblioteca' ('the library') If you are trying to say "There are 100 books in the library." the Spanish is "Hay ciento libros en la biblioteca."
"There is" or "there are".
How many books are in the classroom? why? what?
There is thunder
"It is sunny."
Where is one?
It means: There are clouds.