https://health.gov/dietaryguidelines/2015 mentions that 400mg/day is the upper bound for moderate caffeine consumption but provides no citation.

I cannot find where the number originates, but there is at least one study @ https://www.sciencedirect.com/science/article/pii/S027869151... which explicitly claims "vidence supported consumption of ≤400 mg/day in adults is not associated with overt, adverse effects."