This mystery can be resolved by googling ‘ld50 caffeine.’ ~750mg is toxic and 10,000mg can kill people

The median lethal dose (LD50) of caffeine is estimated between 150 to 200 mg per kilogram but reports of lethal intoxications have been made with doses as low as 57 mg per kilogram. Personally, 300mg is pretty much as far as I want to go.