It was asked facetiously on the TV show Buffy the Vampire Slayer years ago, but surprisingly I have been unable to find a definitive answer on any of the common reference sites (such as Dictionary.com), though that did inform me the word is of Greek origin, then was migrated into late Latin and further into Middle English.
Apocalypsi?
Apocalypses?
Apocalypsis?
How would the plural form be properly pronounced?
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.
Unless I'm mistaken, there can be only one apocalypse, one in which--and through which--evil and evildoers are defeated once and for all. Consequently the cosmos will be restored to its original pristine state and can never thereafter be spoiled by the ravages of sin and evil.
One of the culminating events of the apocalypse is the Battle of Armageddon, which takes place in the Valley of Megiddo (alternately known as the Valley of Esdraelon, or the Jezreel Valley), a geographic area about 60 miles north of Jerusalem, where the forces of evil amass against the forces of good and are defeated in the battle to end all battles (see Revelation 16:16). The valley has been the scene of several decisive battles in ancient times, going as far back as the 15th century B.C.
There can be (and are), however, many different apocalyptic scenarios. In Western culture, the most famous scenario is the one contained in the last book of the Bible, the Revelation of Jesus Christ, which was written by the apostle John, late in the first century of the Common Era.
In other words, there is, theoretically speaking, only one apocalypse; hence, there is no need for a plural form of the word. If there were a plural form, however, I would opt for apocalypses.
