What is the plural form of 'apocalypse'?

Merriam-Webster's Eleventh Collegiate Dictionary (2003) suggests that the original apocalypse wasn't conceived of as a one-time-only armageddon:

apocalypse n [ME revelation, Revelation, fr. AF apocalipse, fr. LL apocalypsis, fr. Gr apokalypsis, fr. apokalyptein to uncover, fr. apo- + kalyptein to cover — more at HELL] (13c) 1 a : one of the Jewish and Christian writings of 200 B.C. to A.D. 150 marked by pseudonymity, symbolic imagery, and the expectation of an imminent cosmic cataclysm in which God destroys the ruling powers of evil and raise the righteous to life in messianic kingdom b cap : REVELATION 3 ["cap : an apocalyptic writing addressed to early Christians of Asia Minor and included as a book in the New Testament — called also Apocalypse"] 2 a : something viewed as a prophetic revelation b : ARMAGEDDON ["1 a : the site or time of a final or conclusive battle between the forces of good and evil b : the battle taking place at Armageddon 2 : a usu. vast decisive conflict or confrontation"] 3 : a great disaster {an environmental apocalypse}

While definitions 1b and (for the most part) 2b refer to Apocalypse as a one-off event, definitions 1a, 2a, and 3 do not. Perhaps most interesting, definition 1a suggests that someone living 2,000 years ago could make a career—or at least an avocation—of writing apocalypses.

The prospect of multiple apocalypses in the definition 3 sense is bolstered by the history of film over the past fifty years, wherein the possibility (if not probability) of countless zombie apocalypses—especially in Hollywood—has become an industry trope.

As for the plural form, it is simply apocalypses, just as the plural of eclipse (which has a different etymology) is eclipses.