There are many, many variables involved. Brain chemistry, liver enzymes, an empty or full stomach, context (a mug of coffee at your desk while working probably affects you differently than somewhere new), smokers metabolize caffeine faster (IIRC), the naringin in grapefruit juice slows down caffeine metabolism, etc.

FWIW, 8-10 cups of coffee is quite a bit more than I could comfortably drink in a day. (Green tea, perhaps.)