How much food can horses eat before they become foundered?

It depends on the individual animal and the food they are eating, as well as how active the animal is.

A very fit animal burning a lot of calories in competition, for example, could eat quite a lot with no risk of founder. On the other hand, a fat pony with little exercise could eat just a couple of pounds of grain a day and founder on it.

Ponies are more likely to founder than horses.

Fat animals are more likely to founder than fit ones.

Animals who suddenly eat too much or who colic are more likely to founder.

Typically, what you see are fat old ponies foundering in spring when the grass suddenly turns green and they pig out.