I had to search about and it's indeed a lot:
"it is quite normal for a horse to poo (defecate) 8-12 times a day and produce anywhere from 13 to 23 kilograms of poo a day."
That's what you get when your primary source of nutrition is very calorie-poor and largely indigestible.
https://en.wikipedia.org/wiki/Great_horse_manure_crisis_of_1...