From what I know there is Mandarin, Hokkien, Cantonese But are there other minority dialect? I've heard one spoken outside the mainlands, can anyone confirm?
yeah there are a lot of them: Cantonese is spoken by people around guangdong（广东）area, Hokkien is spoken by people around fujian（福建） area. There are also Mongolian Language spoken by inner mongolian （内蒙古） people, Tibetian Language spoken by tibet （西藏）people. These are the most famous one i believe, there are still a lot of them since china consist of many clan and area. Every clan and area have there own language
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.
There are many, many dialects. Most individual provinces, regions, cities, and even parts of cities have distinct dialects.
There are nine main varieties of Chinese.
Firstly, we have Mandarin. It has the sub-branches (all of these can be divided into many distinct dialects) of Northeastern Chinese (Dongbeihua), Beijing, Ji-Lu (of Hebei and part of Shandong), Jiao-Liao (rest of Shandong and the Liaodong peninsula of the Dongbei), Lower Yangtze (Nanjing area), Central Plains (Henan, Shaanxi, northern Anhui and Jiangsu, southern parts of Xinjiang, Shandong, Gansu, Hebei, and Shanxi), Lan-Yin (Gansu, Ningxia, and Xinjiang), Southwestern Mandarin (Sichuan, Yunnan, Chongqing, Guizhou, parts of Hunan, Guangxi, Shaanxi, and Gansu), and Huizhou (a few counties in Anhui, Zhejiang, and Jiangxi).
Secondly, we have Yue (Cantonese) divided into its major subgroups Yuehai (Hong Kong, Macao, Guangzhou area), Siyi (Taishan), Gao-Yang (Maoming, Yangjiang), Yong-Xin (parts of Guangxi), Gou-Lou (border areas of Guangxi and Guangdong), Qin-Lian (Qinzhou, Beihai and Fangcheng), and Wu-Hua (Wuchuan and Huazhou).
Pinghua is another group of Chinese (sometimes classified as Yue), spoken mainly in Guangxi province, and has elements from the Zhuang languages in the area. Its two main dialects are Northern Pinghua (Guilin) and Southern Pinghua (Nanning) which are not mutually intelligible.
Min Chinese is a lot more complicated. Min, first of all, has several mutually unintelligible varieties, namely Northern Min, Central Min, Eastern Min, Pu-Xian Min, and Southern Min. Northern Min, spoken in the Jian'ou region, has mostly mutually intelligible varieties. Central Min has the sub-dialects (all located in the Sanming Prefecture) of the Sanming dialect (urban area of Sanming), Yong'an (Yong'an, Sanming) and Shaxian (Sha County, Sanming). Eastern Min, also known by its representative dialect Fuzhounese (Fuzhou), Fuqing (Fuqing, Fuzhou), Fu'an (Ningde), Manjiang (across Zhejiang, primarily in Wenzhou), Longdu and Nanlang (both spoken in Guangdong). Then there is Pu-Xian Min, with the Putian (Putian) and Xianyou (Xianyou) dialects. Southern Min, known by the name Hokkien, has many mutually unintelligible dialects, among them being the "standard" of Minnan speech, the Quanzhang dialect (Quanzhou, Zhangzhou, Xiamen, Taiwan), Teochew (Chaozhou, Shantou, Chaoyang), Longyan (Longyan), Zhenan (vicinity of Wenzhou), Datian (Datian, Sanming), Sanxiang (Zhongshan, Guangdong), Hailufeng (Shanwei, Haifeng, Lufeng), Leizhou Min (Leizhou), and Hainanese (Haikou, Wenchang). Another group of unclassified Min languages, Shao-Jiang, has the subdialects Shaowu and Jiangle and is spoken in these cities, respectively.
Wu Chinese is also very diverse. Wu dialects include Taihu (Shanghainese, Suzhounese, Ningbo, Hangzhou, Wuxi, Changzhou, Jiangyin, Qi-Hai, and Jinxiang), Taizhou (Taizhou), Oujiang/Wenzhounese (Wenzhou), Wuzhou (Jinhua), Chu-Qu (Lishui, Quzhou, Shangrao and Yushan in Jiangxi), and Xuanzhou (Xuancheng, Anhui).
Hakka is another group of dialects spoken in both Taiwan and the mainland. In Taiwan, the main dialects of Hakka are Sixian, Hailu, Dabu, Raoping, and Zhao'an. In the mainland, the prestige dialect is the Meixian dialect (Meizhou), and other varieties include Wuhua, Xingning, Pingyuan, Jiaoling, Fengshun and Longyan.
Gan Chinese is a group closely related to Hakka. Dialects include Nanchang (Nanchang), Yichun (Yichun), Ji'an (Ji'an), Fuzhou (Fuzhou, not the same Fuzhou as in the Eastern Min language), Yingtan, Daye, Leiyang, Dongkou, and Huaining.
Another group of Sinitic varieties close to Gan is Xiang Chinese, spoken mainly in Hunan province. Xiang varieties include Chang-Yi (Changsha dialect), Lou-Shao (Loudi dialect), Chen-Xu, Hengzhou, and Yong-Quan. There is also an interesting distinction between New and Old Xiang.
Jin Chinese is a group of Chinese varieties spoken in Shanxi and Inner Mongolia. Its dialects include Bingzhou (Taiyuan), Lüliang (Lüliang), Shangdang (Changzhi), Wutai (Wutai County and Central Inner Mongolia), Da-Bao (northern Shanxi & Baotou), Zhang-Hu (Zhangjiakou and Hohhot), Han-Xin (southeastern Shanxi and adjacent areas of Henan's Xinxiang and Hebei's Handan), and Zhi-Yan (Zhidan and Yanchuan in Shaanxi).
This is 86 dialects in total, and this is not even accounting for the many cities that have distinct dialects. Although both grouped under Southwestern Mandarin, the Kunming dialect and Chongqing dialect are distinct dialects.
Hope this helps. Sorry for the long answer.