Too much coffee?

Now here is a question! Me & Neil drink loads of coffee to keep us going through the 80+ hour week and as it gets closer to the first Monday of the month (we close the restaurant the first Monday 0f each month so we can have a day off) I'm getting more stupid!! Can you drink too much coffee? How much is too much? I'm going to count how many cups I drink today and record my stupidity!! I've had to switch from real coffee to instant (it gives me a bigger caffeine buzz) . When I say stupid I mean -last night it took my brain a good 45 seconds to remember what a Russian coffee was & I had to go back over to a table and check if they wanted coffee with or after dessert, even though I had written it down on the docket as with dessert and was right, these are things which my brain usually computes & spits out for me before I'm even aware that I need to know them!! So now through a caffine haze I'm doubting my self...... What was I saying.... oh yes.... time for more coffee.