Herschel Walker supposedly did between 2500 and 3500 pushups per day during his playing days and claims to do 1500 pushups and 3000 situps every day even now in his 60's. By no means should that man be considered a normal human being though.
It's especially impressive for him since he's like 220 lbs. If you weigh 160, pushups are a lot easier. If you weigh 280, they're almost impossible.