The entry posted by AletheiaRivers hits the problem right on the head. It is a common misconception among Westerners that Taoism is a specific spiritual doctrine. Although this is true in that there is a Taoist orthodoxy, the term Taoist is often used to refer to many types of people also called Taoists. Take, for instance, the adherents of the philosophical principles of Lao-Tzu, Chuang-Tzu, Lieh-Tzu, and the like. These people were not Taoists in the way that the Pope is Catholic. They weren't really even called Taoists in their time, and didn't consider themselves to be Taoists. There was also the Hsien Taoists, which were much different from the philosophical Taoists and the orthodox Taoists. Their goal as Taoists was to achieve immortality through a series of hygenic, dietary, and ritualistic practices.
Furthermore, even Confucius wrote about the Tao. Though, I believe few people will call him a Taoist, as Confucianism is more likely the appropriate categorization.
To ask how many Taoists there are is like asking how many followers of Abrahamic Religions there are. Although one might be able to give an approximation, much more accuracy is gained by specifying the particular variety that is in question. An answer may be easier to achieve if you specify what type of Taoism your interested in...