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 6-10 Keurigs, but you’d need them on 4-5 (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.
I get real Technology Connections-guy vibes from this response. Thanks for the math stranger :)