Not as many as you think, a water heater only has 3 times the electrical capacity of a standard wall outlet. So probably less than 6 Keurigs, but you'd need them on three (or more) separate circuits otherwise you'd blow the breakers.
If we're looking into their heating capacity they should be able to heat approximately 7 and 1/2 gallons of water an hour. A lower end water heater can supply about 85 gallons of water per hour so you'd need about 11 of them to meet a small house capacity.
If we're looking at their water holding capacity and power consumption. The average house has a 40-60 gallon water heater and a Keurig has a 48oz reservoir. You would need 107 to get to a 40 gallons capacity. When heating they use 1500 watts according to the Internet, so you'd need 160,500 watts (or 1,345.75 amps) of Keurigs to be the equivalent of a low end water heater for a house. The average 40 gallon heater uses between 4500 and 5500 watts.
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.
I read once that if you’re trying to get some creativity juices flowing, removing unnecessary details will allow for more abstract thought. Like “write a story about an ogre” would lead to very different stories than “write a story about an ogre doing pushups”, even if the latter is probably arguably more funny.
So that was my thinking here about being a bit vague, I was specifically talking about tankless water heaters which got us on the subject though.
Oh I totally get you, and it leads to more interesting perspectives and discussion for sure. I don't mean to insult your question by calling it vague - the person was just saying they didn't know what to believe among multiple answers and who was right, and I was pointing out that a vague question has multiple different answers. Didn't mean it to come off as insulting your question.