I would think about 20% of worlds population do not believe in a god/creator.

Some areas are more atheistic than others eg US is about 15%, UK about 40%, Scandinavian countries and Japan 80%+.
I once met a Bahraini Atheist here in Qatar but I am sure that the % in the Gulf is very low.

On your point about knowledge and conviction: I think the more one learns about the religion one is born into, different religions, Science and Evolution the more skeptical of religion one becomes. One needs to know about both science and religion to begin to weigh up which is correct. Studies have shown that the more intelligent one is, and the more one has had access to good quality education, the more likely it is that you will hold an atheistic world view. Of course this data does not imply that religious people cannot be intelligent.

"Deaths in the Bible. God - 2,270,365
not including the victims of Noah's flood, Sodom and Gomorrah, or the
many plagues, famines, fiery serpents, etc because no specific numbers
were given. Satan - 10."