Twenty-three horses ran in a rough-and-tumble edition of the Kentucky Derby in 1974, and that was too many, track officials decided before the 1975 Derby, capping the maximum number of starters at 20, though a court injunction permitted a 21st runner in 1981.
It wasn’t until 1999 that the Derby drew more than 20 entrants again and the 20-horse maximum, determined by graded-stakes earnings, had to be invoked, but since 2004, a full Derby entry box of 20 horses has become the rule.