you dont need a large scale farm to use tons or hundreds of bales.
3 horses will eat ~350kg medium round bale in a week. 3 cows will eat slightly faster than that. 12 goats/sheep will eat a bale a week. They will eat more in cold weather (over 30% more in winter from my experience) or when producing milk.
1 medium round bale is about 15 square bales. 1 big round bale is about 2x medium bale. I'm not sure about big square bales, but id guess about the same as a big round.
You would expect around 15% wastage in a decent feeder. Youre also going to discard some hay due to it not being of consistent quality, etc. Its also quite common to discard the first "layer" of many round bales if they have been weathered/stored incorrectly - especially when using as horse feed.
That said - my understanding is that these sellers have that total quantity to sell. You will likely be able to buy small quantities. Some people go and pick up their hay weekly from producers as they dont have suitable storage.
EDIT: Weights vary - the weights given above are about average from my experience, but vary depending on moisture content, types of grass, how tightly the bale has been compacted, etc.
