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][1], and it's still labeled as uncountable, with no plural form given. And yet, this [Wikipedia page][2] 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*?

  [1]: https://en.wiktionary.org/wiki/leave_of_absence
  [2]: https://en.wikipedia.org/wiki/Leave_of_absence