What's a lot? Excess caffeine can lead to irritability, insomnia, panic attacks, anxiety, tremors, agitation, GI distress including esophageal reflux, cramps, indigestion , diarrhea, muscle twitching, cramps, increased sensitivity to to low blood sugar, palpitations, funny heart rhythms, and more. Excess is a relative thing. If you get the above, you're drinking too much.
Caffeine. Most people can drink the amount of caffeine in 6-8 cups of coffee per day. Over 600 mg of caffeine. But those who use this amount often work up to this dose or higher. Others can only take one or two cups a day or they become anxious, jittery, have worse irritable bowel symptoms, fibrocystic breast pain, problems with sleep, or gastritis from the acid. Disease no, but irritation yes.