I don't know the exact number but I do know that the number is WAY too high.
A lot, go figure lol
50,000 horses die each year because of abuse and abandoning by the owners.
on average 1 human year equates to about 6.5 for a horse until it reach 4 years of age then it drops to about 2.5 per one human year
Around 10 to 20 the last few years.