It's enough to treat 1250 pounds of horses because the horse dose is only 1 mg per 2.2 pounds. The chicken dose is 10-20 times that, it's 10 mg/kg to 20 mg/kg.1 tube which indicates it treats up to 1250 lb body weight. I would think one tube is more than enough.
If you have 18 chickens that weigh 6 pounds each, and one that weighs 10 pounds, that means you flock weighs 118 pounds.
118 / 2.2 x 20 / 77.5 = 13.84 ml
One tube = 7.35 grams = 7.35 ml
You will need two tubes to treat a 118 pound flock.