Possible interpretations:
* Agatha Christie's complete works: This is the most likely interpretation. Agatha Christie wrote 66 detective novels, 14 short story collections, and six novels under the pseudonym Mary Westmacott. This would make a total of 86 books.
* A specific collection published by a certain publisher: In this case, the number of books would depend on the publisher's selection and the scope of the collection.
* A fictional collection: If this is a hypothetical collection, the number of books could be anything the creator chooses.
To get a precise answer, please clarify what you mean by "Agatha collection."