Hens is totally correct as well
So does kana mean a chicken or a hen?
Chicken: bird of the species Gallus gallus domesticus.
Hen: female bird of said species.
It means either. A male chicken is a rooster or a cock. Kukko in Finnish. Also the brand of a popular Finnish beer, as an aside.
How would I say, "I have eight chickens, five of them are hens"?
Minulla on kahdeksan kanaa. Kolme heistä on kukkoa.
You don't know that...the three of them could be chicks! ;-)
"Minulla on kahdeksan kanaa, viisi niistä on naaraita/naaraspuolisia."
Does the etymology of yhdeksan and kahdeksan have anything to do with the fact that they are one before ten and two before ten respectively? Like latin undeviginti and duodeviginti meaning one before twenty and two before twenty?
Yes, it's essentially from an old way to say "two are not there" or "one is not there".