Yes, a horse can eat too many apples - but it would have to be a fair few. Do unripe apples cause colic in horses? They can cause stomache aches in people can't they? But I'd say 5 - 8 apples spread out over the day isn't going to hurt the average sized horse.
We give our horses 2-3 carrots per day and none have died yet!
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare