Okay, you intrigued me again. I did a little looking on the internet. A day horn is just a smaller powder horn to carry around for shooting. But it never gave an average for how much powder it should hold.
So, how many shots can you get out of a day horn? Knowing o course there are different charges for any multitude of reasons. A range is okay, or just an average.
The discussion of Day horns is a bit of a hot topic in some black powder circles. Almost a part of a historically correct topic. The theory goes that most BP guns back in the 18th century were most likely using 2ff or 3fff powder in both the barrel and the flash pan. There were no 4ffff and finer powders available for many years to come. When muzzleloading was reborn in the 1930's there were smaller horns being used to carry a seperate finer powder to prime the pan. After 3/4's of a century of use since ML's rebirth it became accepted as fact.
Smaller original horns are found to this day. But some of the antiques had a charger or small measure attatched to them. This made no sense, in that you wouldn't be measuring a 60 or 80 grain charge for priming. So the theory out there now is that once things settled down and areas became safe...why would a hunter need to carry something large and bulky that was also heavier then necessary. One that won't get tangled in the puckerbrush. A day horn can easily slip into a pocket. If your intending on hunting and coming home that day...then the larger horns that carry a 1lb of powder make no sense.
There is also the theory that some full sized horns were cut down as the thin end of the horn cracked. Waste not, want not. This is evidenced by the scrimhaw being cut off and not being complete, on some horns