20 to 25, most lay daily, some breeds less. Are you sure they are hens? Could a small preditor i.e weasel etc be getting to your eggs. A human egg thief? I have 7 hens 2 years old and I get 5 to 7 eggs a day.
Costco has good prices on nexguard and revolution, might check their price on comfortis. Am using Frontline now but have an Rx for revolution if it fails.