The number of books that make a library is not strictly defined and varies by perspective. Common benchmarks are:

• 1000 books is often cited as a popular milestone to consider a collection a library.
• The American Library Association (ALA) suggests around 5000 books for a full library collection.
• Some consider 500 books enough to "feel like" a library.
• For school libraries, ALA recommends a minimum of about 15-20 books per child, so for a small school of 100 students, around 2000 books.
• Smaller collections, even a few hundred books thoughtfully curated, can be considered a personal library by many.

So, the number of books that constitute a library varies from a few hundred to several thousand depending on the context, size, and purpose of the library.