Complex sentence: Brian acquired a legendary status at the time of his rediscovery in the 1950s and 1960s for the 32 symphonies he had managed to write, an unusually large number for any composer since Haydn or Mozart, and of which six were completed after the age of 90.
Split into simpler sentences: Brian acquired a legendary status at the time of his rediscovery in the 1950s and 1960s for the many symphonies he had managed to write. By the end of his life he had completed 32, an unusually large number for any composer since Haydn or Mozart, and six of which were written after the age of 90.