how much light needed into the coop? How much is too much?

Ventilation means open airflow- not covered in glass. I believe the general rule is 1sq ft of ventilation per bird minimum.
You can add more if you live in a warmer climate, or removable covers on parts for cold/inclement weather- but make sure to maintain the minimum ventilation. It should be in an area not to cause drafts on the birds.

 

Well, mostly chickens should be only sleeping in their coop except in inclement weather, so they really only *need* enough natural light to tell them that it's morning. If you are in a northern climate, I suppose passive solar heating in the winter might be something else to use windows for.

Ventilation is something altogether different, as Paganrose explained above.

 

If you've got enough ventilation, you may not need windows at all. Also, windows are an ineffective source of ventilation, especially if they must be kept closed during stormy, rainy weather.
A big roof overhang, and lots of eave venting or leaving the upper portion of a few walls open with hardware cloth covering works well.

 

Light is good. So adding that piece is good idea.

 

Lots of light in coop is good, for both chooks and keeper.

Top hinged windows that swing open to the outside,
secured inside with hardware cloth against predators,
can be left open in rainy weather for ventilation as well as light.

Can depend on climate and site orientation too tho.
Huge bank of windows on the south side of a coop with no shade in a hot climate could be a chicken killer.