How many christian countries are in the world?

None. Due to the nature of freedom of choice, there is no entirely christian labelled country.
 

Answer

Many countries have adopted their laws and parliament based primarily on Christian principles and in that way can be described as predominantly Christian. Also, if the majority of the population are either church-going Christians, or regard themselves as believers will suggest a Christian country. As an example, in the UK in the last census, around 80% of the population declared a belief in God, despite only around 10-15% of the population attending church. There are many muslims, Buddhists and Hinduss liviing in the UK, but, as the majority of the population is Christian, the country can be classed as Christian. In the same way, Saudi Arabia is an Islamic state, based on Sharia Law but there are small numbers of Christians living there.

If this is the definition of a Christian Country then all the countries of North and South America and Europe are Christian. Many countries in Asia and Africa are also Christian, as are most countries in Australasia - notable Australia, New Zealand and many South Pacific island countries.