Gas or electric?
A high-capacity gas tankless typically has a 200,000 BTU burner, and may require a 3/4" gas supply line. There are also venting requirements.
I have electric. The common "fear" of electric tankless is the huge electric demand they can pull . . . which is true at maximum capacity. Mine is rated 120 amps @ 240 volts, 28,800 watts. But, it *rarely* needs to run at full capacity.
See, the trick about tankless is not to OVERHEAT the water for the task at hand. What-the-hell-for heat it to 120°F or 125°F or 130°F and then temper it with COLD water for a shower? Who could stand under a stream of 130°F water, so what for spend the energy to create it?? Current-technology tankless units throttle the electric elements or gas burner as needed to meet the setpoint based on the incoming water temp and flow rate. By running hot water at moderate rates and keeping temperatures reasonable, high efficiency can be attained.
I have my unit set at 102°F as the normal setpoint, for showering. No need to mix cold with it, it's NOT OVERHEATED. I don't run the water very fast, generally about 0.7 GPM flow rate, but sometimes a bit more. With incoming water temp of between 80°F and 83°F (south Texas in the summer, weeeeooo!), it typically is below 12% of capacity. I've seen it at 7%. 7% of 28,800 watts is a touch more than 2,000 watts, 500 watts more than a typical portable space heater. I could stand under that shower ALL DAY and never run out of 102°F water.
I raise the temp for specific tasks, but that generally is only washing clothes and filling the jacuzzi tub. I don't raise it for dishwashing, my DishDrawer always heats to specific temps. The last time I ran 140°F for a washing a load of whites, the demand was 67% of capacity, almost 20KW, but fill time for the the F&P's EcoActive wash isn't much.
Another concern about tankless is "time to hot." True, there is a brief delay while the unit gets fired up and adjusts to the conditions. I can't say about gas, but my electric typically takes about 20 to 25 seconds to settle down, a bit longer if the temp is raised toward the max. I'm basing that on performance at the washer, which is the closet point of use to the unit. The remainder of the "time to hot" at the point of use is for purging standing cold water in the hot supply line, which is the same as with a traditional tank unit.
As for running multiple concurrent hot water uses, it certainly can be done. Performance depends on all the variables -- incoming water temp, setpoint, and flow rate. High setpoint, low incoming temp, and high flow rate, it likely would have trouble and perhaps could not meet the setpoint. There WOULD be some heated water, just not to the setpoint. I could get 102°F to four or five faucets at the same time. I can fill both whirlpool tubs with 112°F or 115°F. I could get 135°F water to probably two faucets in the summer. 140°F to one in the winter.
The only thing that has changed about my hot water usage is that I'm more
conscious of it now. Do I
really need hot water for that task, how hot does it need to be, and how fast/how long does it really need to run?
