How many different dialect known to exist in the Chinese language and what are they?
There are 7 kinds of dialects mainly: 官话(Mandarin-Similar Dialect)、吴方言(Wu[South Jiangsu, North Zhejiang and a part of Anhui]Dialect、赣方言(Jiangxi Dialect)、湘方言(Hunan Dialect)、闽方言(Fujian Dialect/Hokkien)、粤方言(Guangdong Dialect/Cantonese)、客家方言(Ke Dialect). Here is a easy picture for you:

But they cannot describe all the dialects. For example, Mandarin-Similar Dialect includes 东北话(Northeastern Dialect), 陕西话(Shaanxi Dialect), 云南话(Yunnan Dialect). They are quite different in Chinese people's opinion. So these 7 kinds have many smaller kinds of dialects, such as 吴方言, has 6 dialects, including 太湖(Lake Tai)/台州(Taizhou)/金衢(Jinhua-Quzhou)/上丽(Shanghai-Lishui)/瓯江(Oujiang)/宣州(Xuanzhou) 片. So, in fact nobody can confirm the exact number of Chinese dialects.
There were no Mandarin Chinese speaker several hundred years ago in some areas, even now, some people in Tibet cannot speak Mandarin well. So there's no dialect in Tibet, Xinjiang and Qinghai.