204 National Olympic Committees are recognised by the International Olympic Committee, and all 204 are represented in London in 2012. However, athletes from the island of Curaçao (which has recently achieved some sort of autonomy after the dissolution of the Netherlands Antilles), and South Sudan (which is also so new that it doesn't have an NOC) are also competing, under the Olympic Flag. So the total could be 206. However, again, some 'countries' recognised by the IOC are not members of the United Nations, such as Hong Kong or Guam, so in some ways the answer to your question depends on how you define a country.
205
greenland
The US has 529 athletes in the 2012 Olympics.
London will host the 2012 Olympics
216 countries will participate in 2012 Olympics.....
380 athletes sailed in Olympics 2012.
there are 32 events at the 2012 olympics.
Slovakia has 46 athletes competing in the 2012 olympics
An estimated 10,500 athletes will compete at the 2012 London Olympics.
204 countries participated in the 2012 London Olympics.
204 countries participated in London Olympics 2012.
An estimated 10,500 athletes will be participating in the 2012 Olympics.
320 gold medals will be given out at the 2012 Olympics.