Sounds exaggerated, unless he had insane tolerance levels or the coffee was very weak. Typically you count ca. 70mg for a cup of coffee, though of course it varies a lot with type. If we assume 50mg, that's 2500mg...
For comparison, weight lifting supplements tends to max out at about 400mg, and from experience if you take those amounts and then don't do a hard workout, you're bouncing off the walls and it's horrible. I find it hard to imagine effectively spacing out 2500mg in a way that'd leave me focused rather than jittery as hell, despite a very high tolerance level, and without wrecking any hope of decent sleep....
Then again, tolerance levels for caffeine varies greatly - at one point I'd often drink a red bull right before sleep.
"Voltaire was also known to have been an advocate for coffee, as he was purported to have drunk it at least 30 times per day. It has been suggested that high amounts of caffeine acted as a mental stimulant to his creativity." - wikipedia
If you follow Wikipedia's source for that, here is what it actually says:
In France, meanwhile, Voltaire was reputedly downing between 50 and 72 cups of coffee a day
Encyclopedia authors shouldn't cut numbers in half. In any case, the reported number varies wildly and I've never been able to track down a source for it. They're probably all made up.
When I first heard this claim, it was about the French author Honoré de Balzac. This claim about Voltaire makes me doubt both. I'm not saying that it's impossible to have two famous French writers habitually drinking an atypically obscene amount of coffee, just that it makes the claim seem more dubious.
That's the story at least. Don't know if it's true.