Every apple pie recipe is lying to you. It is lying by virtue of being a recipe, of pretending to tell you that the same amount and combination of ingredients will work without fail. This is untrue. Apples vary; they vary from species to species, and from one batch to the next.
The amount of sugar you want to use, the amount of flour or corn starch - or if you even bother to include either flour or corn starch - depends on your apples. No recipe can tell you the exact proportion with 100% accuracy, because no recipe knows what your apples are like. Heck, YOU don't even know what they are like until you start slicing them.
The juicier your apples, the more flour (or corn starch) you want to add. Flour helps sop up the juice, and convert it into the delicious sludgy fruit gravy that is the hallmark of a good apple pie. The specific amount to add will depend on the specific amount of juice you are getting.
If your apples are dry - as they often are this time of year, having been harvested all the way back into last fall - then you can skip the flour. If they are VERY dry, you may even want to add a sprinkle of water or juice, a few extra dabs of butter, an extra splash of lemon juice, or all three. There is nothing sadder, less appealing, more uncomfortably "homemade-looking," than a dry apple pie.
The amount of sugar will also vary depending on your apples. The sweeter the apples, the less sugar you need. I have made pies with apples so sweet, all I had to do was dust them with a few tablespoons of sugar, and even that was probably unnecessary.
Sugar does more than sweeten your pie, though. It helps suck the juice out of your apples (maceration) to make that yummy filling. The longer the sugar sits on your apples, the more juice is sucked out. If your apples are on the dry side, use a little more sugar just to be sure. And try not to let them sit around very long after they have been sugared, lest you end up with leathery pieces of apple.
The dry ingredients should ideally be measured out after the apples have been sliced. Err on the side of adding too little, because it's easier to add more. Mix everything around with your hands and see how it looks. You will quickly get a feel for how it ought to be, and adjust your ingredients to suit.