Why are there 12 men on a Canadian football team?
Although both the National Football League and the Canadian Football League require seven men to be on the line of scrimmage, the CFL allows five (to the NFL's four) players in the backfield. All five, plus the players lined up at each end of the line of scrimmage, are eligible receivers. This formation is easier to accomplish in the CFL because regulation fields are 15 yards wider then NFL fields.
It's just one of the several differences between the two sports. To learn more about the CFL check out their web site.
—The Fact Monster