On average a woman ovulates once per month. When a woman ovulates her cervix, which is usually closed, opens slightly to allow sperm to enter the uterus for fertilization. Of the initial 300 million human sperm on average that start out, only about one million human sperm will typically make it into the uterus, and of these only a small fraction make it to the egg.
What might happen when a horse cums inside a woman who happens to be ovulating with a cervix open? First the the average horse ejaculates 8 billion sperm, an the sperm shoot out the penish with a great deal of force. Only .33% of human sperm make it past the cervix. If we conservatively assume that 1% of th horse sperm are able to pass through the open cervix, helped to pass through by the much greater force of the horse ejaculate as it shoots into the vagina, this amounts to 80 million horse sperm making it into the uterus, 80 times more than the amount of human sperm that make it into the uterus.
How would this equate to amount of sperm reaching the egg?
200 human sperm are able to reach the egg. If the horse sperm are able to reach the egg with the same success as human sperm, this would amount to over 5000 horse sperm which make it to a single human egg.