No government census in Turkey records sect within Islam, so every published Alevi population figure is a scholarly or advocacy estimate; the widely repeated ten to twenty million range is quoted across sources without a shared reference year.
What would resolve this A dedicated demographic survey fielded in Turkey that asked about Alevi identity specifically, dated and methodologically documented, would let a real time-stamped estimate replace the undated range in circulation today.
OpenDavid Shankland, The Alevis in Turkey: The Emergence of a Secular Islamic Tradition