If your average house only has one person showering at a time an 18 kW 3.51 GPM Tankless Electric Water Heater would work. It would cost about $500.

To replace that with Keurigs would take some work. They draw 900 to 1500 watts depending on model. So it would take between 20 and 12 Keurigs to match the power draw. I’m not sure what the inlet/outlet tubing diameters are but it seems some replacement parts labeled 1/4" so that’s half a gallon per minute max. You’d need 7 or 8 running in parallel to move enough water. Probably 21 900 watt Keurigs running in groups of 3 would come closest.

I estimate about $2000 in Keurigs plus another few hundred dollars in fittings and a lot of time to reproduce a purpose built $500 water heater.

And of course Keurigs break frequently and you’d definitely void any warranty.

Good luck!