I've only checked the horns of plenty on shards where I had a place to lock down the coffee plants and let them grow. It seemed like I had better luck getting the coffee plants and pigs on a spit if I used the character who originally received the horn to empty it versus having one character click thrown a whole pile of horns of plenty.
These were the results from the initial round of emptying all the horns of plenty (10 charges each):
Atlantic: 24 horns, 4 coffee plants, 6 pigs on spits.
Balhae: 24 horns, 7 coffee plants, 5 pigs on spits.
Formosa: 21 horns, 5 coffee plants, 6 pigs on spits.
Great Lakes: 14 horns, 2 coffee plants, 6 pigs on spits. (Gave away several full horns on this shard and the ones that were left I collected and had one character click through them all.)
Hokuto: 17 horns, 7 coffee plants, 6 pigs on spits.
Lake Austin: 28 horns, 9 coffee plants, 7 pigs on spits.
Origin: 36 horns, 17 plants, 9 pigs on spits.
Siege Perilous: 5 horns, 1 coffee plant, 3 pigs on spits.
Sonoma: 24 horns, 8 coffee plants, 9 pigs on spits.
Yamato: 26 horns, 6 coffee plants, 8 pigs on spits.
Total these 10 shards: 220 horns (2200 charges), 66 coffee plants, 60 pigs on spits.