The last opus number that Chopin himself used was 65, allocated to the Cello Sonata in G minor.

Chopin expressed a deathbed wish that all his unpublished manuscripts be destroyed. However, at the request of the composer’s mother and sisters, his pianist friend and musical executor Julian Fontana selected 23 unpublished piano pieces and grouped them into eight opus numbers (Opp. 66–73). These works were published in 1855.

In 1857, 17 Polish songs which Chopin wrote at various stages of his life were collected and published as Op. 74—the order within that opus having little regard to the actual order of composition. Two other songs were published in 1910.

Works that have been published since 1857 have not received opus numbers. Instead, alternate catalog designations have been applied to them.

Source: Wikipedia