There are two answers to this question:
Firstly is the epistemological answer which is roughly 22:
http://www.adherents.com/Religions_By_Adherents.html
Secondly the ontological answer is probably billions. Every persons belief is unique. Whether or not any two given people label themselves as 'Christian', their views or practice of the religion may differ slightly giving a variation of the religion 'Christianity'; furthermore, atheists tend to believe whatever they see the most logical and debatebly each of these individual beliefs could be classified as a religion.