ADHD people drink coffee as a form of self medication. Caffeine is an absolutely terrible and barely working stimulant for that purpose, so we end up drinking a lot to get some effect, or at least trick ourselves.
A cup of coffee is 6oz. I drink like 6-8 a day. I do however stick to average roasts, rather than robustas or light "breakfast" roasts.
My brother has that brain worm (that I used to have) where using a medication is "bad" so instead he drinks like 5 giant energy drinks a day.
My father drinks a similar amount of coffee as I do per day, and often drinks coffee as a late night relaxation aide. Stimulants can be "calming" to some people who have a certain genetic mutation and often have ADHD
The first dose of Ritalin I took, I immediately fell asleep and took a 2 hour nap.
But to give a real answer:
On workdays I have about 20-40 fl oz of coffee during the morning. I stop all stimulants at noon so I can sleep.
On non-workdays I have 1-3 normal sized mugs of coffee in the morning, just because I like it.