has a few small vents, but thinking we could add some larger ones to the door and sides (the top of the basement is slightly above ground).
Assuming that this outside access area is completely sealed off from the human inhabited parts of the house the issue is going to be ventilation.
You have to think square FEET, not square inches. 10 square feet as a minimum for 10 birds, maybe more depending on climate. Given that basements are usually damp, I'd expect to have to at least double that because dry chickens are healthy chickens.
Here's my article about it: https://www.backyardchickens.com/articles/repecka-illustrates-coop-ventilation.77659/