A recent piece in the New Zealand Herald pondered the question of how many people in New Zealand speak Chinese.
Raymond Huo of the NZ Chinese Language Trust claimed that "Treating Mandarin, Yue or other Chinese dialects as independent languages is deeply flawed," because "it is similar to making statistical inferences about the difference between northern English, Oceania English and Indian English, or ... between pub talk and the King's English."
Actually, to better understand the linguistic situation in China, think of Europe as a single country, where everyone speaks the language of 'European', with the various different dialects of 'European' - for example French, Spanish, Italian, German, Swedish - all tracing their roots back to an ancient common ancestor that no longer exists.
Furthermore, imagine that the only language used in schools, on television, in movies and in government offices was German and that the only written language available for everyone to use was itself largely influenced by spoken German. Simply substitute Mandarin for spoken German and written Chinese for written German and we can clearly understand the nature of the problem.
Linguists often say that the question of whether two varieties of speech are in fact different dialects of the same language or rather different languages altogether boils down to whether speakers of each variety can understand each other.