How many chirstains are in this world?

It is not possible to know, with any accuracy, how many Christians are in the world, but the most common estimate is about 2.2-2.3 billion.

One of the problems with this estimate is that it appears to include many people who live in "Christian countries" but are not Christians themselves. China is an example of another problem with many estimates: I have seen a conservative estimate of 11 million Christians in China and an optimistic one of over 60 million Christians; with some outlier estimates even higher, proving that we do not really know.