the link says:
Included measuring cup makes dosing easy
You'll use 12 ml per gallon of water as a preventative, or 24 ml to treat an active illness. The integrated measuring cup makes it easy to dose their water with the right amount of product.
so it could treat 20-10 gallons
it's not pure oregano, first 2 ingredients are carriers ,  but has other good herbs in it, so that's why it uses more than a drop
RopaPoultry Complete Ingredients:
Emulsifier (E484) propylene glycol, water, oregano, cinnamon, pepper, nicotinamide, pantothenic acid, milk thistle, artichoke, peppermint oil, eucalyptus oil, thyme oil
eta a previous discussion on this product  
https://www.backyardchickens.com/threads/ropapoultry-oregano-oil-supplement.1048687/