I don't think you can give them too much ACV. Meyer Hatchery told me to give my chicks half ACV and half water! Because of that, I would probably give them more than just a drop, maybe 5.
That is not correct, because as soon as you "overdose" ACV they will not drink the mixture which will quickly lead to dehydration, constipation etc . Even more harmful with brooder chicks that are fed pellets/crumbles and live in dry surroundings caused by the dry heat emanating from heating lamps or heating pads. They therefore need to drink much more to stay healthy than chicks with a mother hen living in a natural garden.