Which opera has more acts, Der Zwerg or Billy Budd?
Der Zwerg:  17, is an opera in one act by Austrian composer Alexander von Zemlinsky to a libretto by Georg Klaren, freely adapted from the short story "The Birthday of the Infanta" by Oscar Wilde. Billy Budd (opera):  Originally in four acts, it was first performed at the Royal Opera House, London, on 1 December 1951; it was later revised as a two-act opera with a prologue and an epilogue.
Billy Budd, Op. 50, is an opera by Benjamin Britten to a libretto by the English novelist E. M. Forster and Eric Crozier, based on the short novel "Billy Budd" by Herman Melville. Originally in four acts, it was first performed at the Royal Opera House, London, on 1 December 1951; it was later revised as a two-act opera with a prologue and an epilogue.