One U.S. customary cup is equal to 8 fluid ounces, but not all coffee cups people use actually equal the amount in a standardized U.S. cup. Therefore, it is impossible to say that all cups equal 8 ounces, or any particular amount.
There are differently sized units that are also called cups, besides the U.S. customary cup. There is the U.S. legal, imperial, metric, Canadian, Japanese and Japanese traditional cup. All of these hold different amounts of coffee. Beyond those standardized measurements, cups vary in size widely. When researchers study coffee drinking habits, they define a cup of coffee as 8 ounces.