UFC 129 set the UFC attendance record with 55,724 spectators.
The event was held on April 30, 2011, at the Rogers Centre in Toronto, Ontario. The stadium setting allowed the promotion to stage its largest live crowd at that time, far exceeding the capacity of a typical indoor arena.
The main event featured Canadian champion Georges St-Pierre defending the UFC welterweight title against Jake Shields. St-Pierre won by unanimous decision. The card also included José Aldo’s first UFC featherweight title defense, against Mark Hominick.
UFC 100 and UFC 200 were also major commercial events, but neither is the answer to this attendance question. UFC 129’s figure is specifically the announced live attendance, not a pay-per-view purchase total or television audience.