Small amounts of feed at one time !
First of all your horses needs. The amount of work (and the season if he is turned out) .
Feeding horses depend a lot of what the horse is doing (working/not working), if he has a padock during the day (pasture), age, etc.
They need forage and, in case of working horses mostly some concentraded feed!
In my opinion, horses must have a lot of hay (good quality and most nutricional) all day long! If they go to the padock and have pasture, then they'll need less in the stable!
If a horse is working 5/6 times a week it should eat at least 3/4 times a day his concentrated feed! the amout variates from feed to feed and from horse to horse (a big horse will eat more, a rich feed is given in smaller quantities)
Then you have some suplements you can give to your horse, for example if he's an old horse he'll need something for the articulations or something else!
All of this depends a lot in what kind of horse you have! If you're having doubts about what to feed him, ask your vet, it's the best person to give advice!
Hope it all helps