The most participants at a Summer Olympic Games celebration is 10,942 athletes, from 204 countries, at the games held in Beijing, China in 2008.

Beijing 2008 hosted 302 events in 28 sports.

Sydney 2000 attracted 10,651 athletes from 199 countries, with 300 events in 28 sports; Athens 2004 welcomed 10,625 athletes from 201 countries, with 301 events in 28 sports; Atlanta 1996 accommodated 10,320 athletes from 197 countries, with 271 events in 26 sports. London is expected to attract an estimated 10,500 athletes from 204 countries, with 302 events in 26 sports.

Note that the sources listed do not generally support these figures (which were taken from the sources listed on each Olympics’ Wikipedia page), including the figure of 10,942 referred to in the record text.