Complex sentence: Even though it may be assumed--given the demand and fashion--that he produced a number of triptychs, only the "Dresden altarpiece" survives, although a number of extant portraits may be wings of dismantled polyptychs.
Split into simpler sentences: Even though it may be assumed--given the demand and fashion--that he produced a number of triptychs, only the "Dresden" example survives. A number of extant portraits may be wings of dismantled polyptychs.