I’m so glad that you asked that question, but there is no simple/easy answer. It’s almost like asking “which is your favorite child?” Some people want the French-style of creamy eggs, some want a soft (but not custardy) egg and others want something firm and dry.
How much milk to add depends on how you want your eggs at the end of the story.
Personally, I’m of the school that doesn’t introduce milk at all. This link from BonAppetit describes how I create my perfect scramble. https://www.bonappetit.com/recipe/bas-best-soft-sc...
But the cooks at Serious Eats recognize that there are different strokes for different folks. Here is a link to their recipes. Scroll down past the photo of the eggs and you will find links to “fluffy scrambled eggs,” “soft scrambled eggs,” and “French-style soft, spoonable.”