Passage: At the end of 2009, the population of Tianjin Municipality was 12.28 million, of which 9.8 million were residential holders of Tianjin hukou (permanent residence). Among Tianjin permanent residents, 5.99 million were urban, and 3.81 million were rural.  Tianjin has recently shifted to rapid population growth, its population has reached 14.72 million as of 2013 end.The encompassing metropolitan area was estimated by the OECD (Organisation for Economic Co-operation and Development) to have, as of 2010, a population of 15.4 million.The majority of Tianjin residents are Han Chinese. There are also 51 out of the 55 minor Chinese ethnic groups living in Tianjin. Major minorities include Hui, Koreans, Manchus, and Mongols.

Question: How many minor Chinese ethnic groups are there?
55