Depends on how many horses you are feeding, and how they are living. A large bunch of horses can eat a round bale per day. Or a half a round bale. Depends on how many horses, and how big the round bale is (can vary between 600 lbs to 1500 lbs). To know how much to feed, you need to know the weight of the round bales you are using, and how much hay your horses need daily, which also depends a lot on the type of horses you have. Many TBs can simply free feed on hay. Many other types of horses will kill themselves if they do that. So you have to figure this out for your own situation.
As walktrot describes, there are dangers in using many of the metal round bale feeders available. Most are actually made for cows, not horses. Some people get away with using them for years, no problems. Other times, problems happen. Pay your money and make your own choice.
For only one, or a few or couple of horses, the way to have the least wastage when using round bales is to unroll it in your barn, and load the hay into a wheelbarrow to deliver it to the individual horses, just like it was flakes off small square bales. Just feed by weight. This is the “high maintenance” of regular horse care, and some people want to reduce work load by using round bales, and want to put the bale out with the horse/s once a week, then limit the intake physically, with nets etc. If you are in a high rainfall area, then a roof over the bale, and a platform to keep it from sitting in a puddle are necessary, plus the tractor to bring the bale out of the barn. So high demand for infrastructure and machinery to produce less daily manual work, but not cheap. For one or a few horses together, I personally go for the manual method, twice a day or whatever you need to do, with a wheelbarrow rather than the wastage of having the whole bale out there for days. With many horses living together, I take a whole bale out, and unroll it on the ground (but I have NO mud issues, ever. I have packed snow in winter, and semi arid weather year round) and all the hay is gone by the next day. I have horses who can eat hay in an unrestricted manner, so this works for me. May not for you. I will not use metal feeders, because of the risk of injury. I do use tire feeders, big ones, both for the loose hay (I feed outdoors on sand) and sometimes for small groups of horses. These tires are BIG, like 6’ diameter, and 3’ high, from logging equipment. An entire 600 lb round bale will fit into one, when you take a saw to one side of the tire and cut that side off, enlarging the opening. The intact side on the bottom holds the bale off ground contact. They are high enough to hold that bale in there snugly. You take the strings off the bale and just drop it in there from the tractor bucket. You figure out how long it will last in terms of “number of horses X number of days, at 30 lbs of hay per day for each horse”. You can do this and put a net over the tire if you like, and if you can figure a safe way to attach the net to the tire. Get creative. There are instances of horses getting INTO big tires like this, and finding a way to kill themselves by doing this, but, IMO, this is less of an issue than with the metal feeders. If you have sand paddocks like I do, I think that the risk of death by sand colic is greater than the risk of death by rubber tire feeder. But your mileage may vary.
If you are new to using round bales, know that many round bales are made for cows, not horses. They may not be horse quality, and you won’t know that until you get into the center core of them and find mold. You may find fermented grass in the center too, which is less of an issue than mold is. But be careful where you buy your round bales from, make sure that the farmer round baled horse quality hay, under low humidity conditions. It has to be VERY dry to round bale and not ferment or mold, drier than for small square baling. Hay made in high humidity environments may not be dry enough to be horse quality in round bales, when it may be just fine in small square bales. Moisture in small square bales can still escape after baling, whereas the round bale centers can be so tight that any moisture can not escape, and rot or fermentation may occur.