How many Scientologist are in each country?

Answer 1

Statistics regarding membership are highly unreliable. Contrary to one's own name or job, there is not official record in the US to keep track of the religious affiliation of citizens. Members of new religious movements tend to be more reserved in their religious affiliation and only share their views within a constricted circle of friend. Most of the statistics that one can find on the internet mostly rely on social media and polls, and fail to provide an accurate answer.
Another consideration is that new religious movements tend to be more localized. For example if we take the number of Scientologists in the city of Los Angeles, and the number of Scientologists in Oklahoma City we could find that the number of Scientologists in Los Angeles is 200 times higher (not an actual number, it is just to provide an example) because of the higher concentration of artists and Scientology Organizations in that area. Nationwide statistics should not be used for New Religious Movements, because they are not territory specific.

The best source of information on membership is from the religion itself. Only Scientology churches can keep track of the exact numbers. They have records of courses and services, which further allows them to separate active Scientologists from not-active Scientologists. They keep records of visits for their mailing lists. For reliable statistics by Scientology, I would suggest reading the Freedom Magazine. You can find the website online or by Google typing "Freedom Magazine".