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There were 277 Canadian athletes competing at the Summer Olympic Games in London 2012. The official London 2012 site, however, has 281 athletes listed throughout all sports.
There are 410 athletes in the Australian Olympic team competing at the London 2012 olympics.
Wikipedia lists Brazil as having 259 athletes competing in the 2012 Summer Olympic Games
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The Australian Olympic Committee sent a total of 410 athletes to the Games to compete in 23 sports
The country was represented by 184 athletes, competing in sixteen sports.
The Independent Olympic Athletes who are competing under the Olympic flag in the 2012 Summer Olympics in London are:Liemarvin Bonevacia from CuraçaoReginald DE Windt also from CuraçaoGuor Marial from SudanPhilipine van Aanholt from the Netherlands.
There will be 75 athletes competing for Jamaicia
Sport brings the world together and over 14,000 athletes from 205 Olympic teams and 170 Paralympic teams will be competing at the London 2012 Games.
I am assuming you mean the 2012 London Olympics. The United States sent a total of 530 athletes to the London Games, 269 of whom are women and 261 of whom are men.
204 Olympic Committees are sending athletes to the 2012 London Games. This is virtually every nation except Vatican City. This also includes territories (such as Puerto Rico or Hong Kong) or unrecognized countries (such as Palestine) competing and the Independent Olympic Athletes, competing under the Olympic Flag.