I never really asked my barn owner, but I'm pretty sure she does for at least pasture. We have 3 different boarding levels. Full Care (Stall), $350, Pasture/Dry Lot $250, and Back Pasture $175. I overheard her say something about some boarders who only stayed one month and moved because board was "too expensive for two horses"...and then went on to say that they paid $300 for both horses to be in the back pasture. So they got a $25 per horse.
There's only a couple of people who have multiple horses. One has two horses and the other has 3 horses plus 2 mini donkeys - not sure what all discounts they get but I know the person with the 3 horses and 2 donkeys has all 3 horses on Full Care and the donkeys share a stall which is open to their lot in the back. It's really none of my business so I never asked what if any discounts they get, but I'm sure if the people from the back pasture got a 2 horse discount of $25, some sort of discounts stands for everyone.
Cinnamon Whiskey 11 y/o 15hh Chestnut AQHA mare, 2'6 Jumpers