That is a really good question and I think the answer is that they don't really know.

In S-A we do census reports, it is not 100%acurate but it gives you a more or less idea, I don’t know if this is implemented in all other countries as well

I'm guessing either prison population or a dysfunctional legal system.

Good question. And to answer it. I will just direct you to an example of a judge refusing to accept a statement of not being Muslim. Egyptian judge to Christian murtaad (apostate from Islam): "If I had a knife now, I would kill you" January 30, 2009 EGYPT (Compass Direct News) – After her arrest at Cairo's airport on Dec. 13 while attempting to flee anti-Christian hostilities in Egypt, convert Martha Samuel Makkar was granted bail on Saturday (Jan. 24), but not before a judge took her aside and said he would like to kill her. According to her lawyer, Attorney Nadia Tawfiq, Judge Abdelaa Hashem questioned Makkar extensively about her Christian faith during the hearing. Makkar, charged with forging identification documents, explained her reasons for her conversion, avowing her Christian faith and repudiating the judge's claims that converting from Islam to Christianity was impossible. "Then he said, `I want to talk with Martha alone,' so we all left the room, and he said to her, `Nobody changes from Muslim to Christian – you are a Muslim,'" Tawfiq said. "And she said, `No, I am a Christian.' He told her, `If I had a knife now, I would kill you.' [Makkar] came out crying and depressed, but at least he gave the decision to let her go free." Makkar, 24, had planned to escape the dangers she has faced in Egypt by travelling to Russia with her family. She says that since converting to Christianity five years ago, police and members of her extended family have threatened her incessantly, the relatives vowing to kill her. Airport security personnel had been notified of Makkar's plans, according to a Coptic rights group. "They had both [her original and Christian] names and maybe a picture before she reached the airport," said Helmy Guirguis, president of the UK Coptic Association. "They did not [arrest her] to apply the law, they did it because of hate for Muslims converting to Christianity. It is like a great occasion to go and arrest some poor lady like her in the airport." After her arrest, Makkar was charged with carrying forged documents and taken to El-Nozha police station. Authorities also took her husband and two children into custody. The identification that Makkar carried listed her religion as Christian and bore the name she had chosen for herself rather than her given name, Zainab Said Abdel-Aziz. Legal conversion from Islam to Christianity by Muslim-born Egyptians, and gaining corresponding legitimate documents, is unprecedented in Egypt. Egyptian law does not provide for a means to legally change one's religion on identification papers. According to Tawfiq, Makkar said authorities held her in a room at the airport, hit her and denied food to her children. "People who convert to Christianity are treated exactly like terrorists," said Guirguis of the UK Coptic Association. "This is not official policy, it's not on paper, it's not the law, but it's what happens." Since Makkar's arrest, she has leveled allegations of sexual abuse and demeaning behavior at police in the El-Nozha station and at personnel of the national security office in Heliopolis. Makkar said she has also suffered at the hands of fellow inmates at Al-Qanata prison, where authorities later took her. "She has some difficulty with the other prisoners in prison," said Tawfiq. "One of them kicked her and tried to kill her; one took the Bible and threw it on the floor, pushed her and tried to make her go back to Islam. But she is strong, she is strong." http://islamsnitemare.blogspot.com/

Hahaha! That is a very funny question. + to you. Well, besides the obvious fact that such a country cannot function, that 'country' will never know how many don't believe in Islam. Best thing to happen about this is the UN comes in and takes a guess.

Every Country has a census to estimate how many people are presented in that country. In which every household has to fill out a form in which they give information about everyone living that house. You have to give your caste, breed, religion etc

I assume that they don't want to know. If asked, they would say 100%.