In 2017, 20 deaths occurred among a total of 8,463 starts over a span of 122 racing days at Santa Anita, according to the most recent figures compiled by the Jockey Club.
https://www.foxnews.com/sports/santa-anita-suspends-racing-indefinitely-after-21st-horse-dies.amp
Odds of a fatal injury in any one race would be around 1 in 500 then.