There’s another saying 34 equating it to a tank less heater.
The original question is too vague - there’s no one to one mapping between keurigs and water heaters. If you’re just trying to heat your houses hot water, any of those answers are valid. So is “1”, but it’d be slow and you’d only get a half gallon before refilling it, and at that point the last batch is cold, but it’s still heating water… . It’s just a question of what you REALLY want and what your constraints are.