this dough is soooo sticky that kneading is practically impossible. How much flour is it acceptable to add (in order to be able to knead the do...

The resting time, known as an autolyse, is a very useful tool to build into your bread repertoire. I use it for every single bread I make. Once the dough has come together, I turn off the mixer and cover the bowl with plastic. The rest allows the gluten strands in the flour to gently expand, taking on more water as they do so, outside the stress of kneading, which tends to squeeze water molecules out of the gluten. After the rest - typically 15-20 minutes - you will be amazed at how much less sticky the dough is. This method works equally well with either a mixer or hand-prepared doughs.

I agree with ChefJune, it always depends. That said, I routinely add a half cup of flour in a recipe that calls for 3 cups, some days a little more.

You could also let it rest for 10-15 minutes and then come back and try kneading again. The extra time to let the flour start hydrating will reduce the stickiness.

There is no set amount. Clearly you need more flour, but only you will know when you've added enough. This is always true for yeast doughs. The amount of flour specified in any bread recipe is a guideline. There are many factors surrounding how much you need at any given time, including the humidity in the room or even outside. The more yeast dough you work with, the easier it will get for you to tell how much to add. Start with a modest amount, because you don't want the dough to be too dry. Softer dough is always preferable to a dry dough.