It will really all depend on the food you are buying. The back of the bag should give you a general guideline. For kittens, it is suggested that you feed them as much as they will eat, so it is difficult to tell exactly how much you will need. I think most people here just free feed their kittens.
If I needed to know how much to buy, I would probably look at the back of the bag and see how much it tells me to feed a kitten (the youngest age at which they include a measurement) and use that as a guide, realizing that the kitten might eat a little more or a little less.
You will find that most people here suggest feeding wet and dry, in some combination. I can tell you that my 10 week old kittens eat maybe a 1/4 can of wet food (medium quality) in the morning and evening, and then maybe 1/2 cup of dry food (high quality) each day. It takes them a bit to get all the wet food down and then they graze the dry stuff. Usually, they eat dry when I'm not here, so I wonder if they like the privacy!