Normally, if a word is generally uncountable ("English vocabulary"), but sometimes it becomes countable to convey particularity or variety ("the many vocabularies of various English dialects"), it'll be labeled as [U, C] in the Oxford Advanced Learner's Dictionary, or as [uncountable or countable] in the Longman Dictionary of Contemporary English. The noun leave is, thus, only uncountable, as it's only labeled as [U] and [uncountable]. I even consulted Wiktionary, and it's still labeled as uncountable, with no plural form given. And yet, this Wikipedia page employs a host of singular and plural forms of leave of absence.

So could leave of absence ever be countable and take a singular form as a leave of absence and a plural form as leaves of absence?