Given the task definition and input, reply with output. In this task, you're given passages that contain mentions of the quantity of a particular object and are supposed to find out the answer for a given question. Craft one correct answer to the question given in input. Make sure your correct answer is consistent with the context, and properly gives the exact number relating to the query.
In your answer, use as few words as possible from the given context. To be less repetitive, please vary your language for each question.

Sentence: The last opus number that Chopin himself used was 65, allocated to the Cello Sonata in G minor. He expressed a deathbed wish that all his unpublished manuscripts be destroyed. At the request of the composer's mother and sisters, however, his musical executor Julian Fontana selected 23 unpublished piano pieces and grouped them into eight further opus numbers (Opp. 6673), published in 1855. In 1857, 17 Polish songs that Chopin wrote at various stages of his life were collected and published as Op. 74, though their order within the opus did not reflect the order of composition.
Question: How many unfinished pieces did Julian Fontana make into eight more opus numbers?
23