TLDR: LOTS and that depends primarily on your camera, lens and size of your green screen.
A good green screen is slightly darker than middle gray. Below is a photo of mine:
To pull a good key you need to have low noise and good separation of the subject from the background (i.e. sharp edges).
So let's say you want to shoot at ISO 100 (low noise) and F5....
Some thoughts about this question.
it seems that fix can be made only by adding more professional light.
Where to start?
The problem you have is noise. Period.
Noise is a product of low light, yes, but where does this low light is originated is the real question.
What aperture are you using and why? what shutter speed and why? and after these ...
Sites like google are not enough to quickly find reference of valuable information on the topics I mentioned above
You live in a privileged era, never, ever in the history of humankind was the information, examples, tutorials, were so easily available.
Where? the internet.
The problem I see without knowing you, but reading your question is that ...