On Wiktionary we read that 'hereafter' has as plural 'hereafters', but OALD says that this plural form doesn't exist. Which is wrong: Wiktionary or OALD?
From the Oxford English Dictionary, under the noun entry for hereafter:
1689 W. Sherlock Pract. Disc. Death (1731) iii. §6. 143 The Reason‥will serve for all hereafters, but will never serve for any Time present.
Evidently, it's a word. Not a widely used word, mind you, but a word nonetheless.
Also, from a Google Book search (under The Complete Works of Thomas Brooks, Volume 4):