Perhaps, they can take a census of the school systems. How many students are registered "Christians" and how many are registered "Muslims". Is that too difficult to count? That could give us an idea or estimate how many in the population there is, notwithstanding the birthing rate of Muslims if they are indeed higher.
There should be the ability to take count of them all by national ID cards, right? I thought to enroll in school in Egypt one needed the ID which lists religion on it....somewhere, in some hot and dusty building in Cairo that info. exists in one place. Maybe.