Given that they are all electric, it would probably be cost prohibitive
as they would need multiple units to achieve a reasonable flow at the
temperature (city water temp vs. outlet temp) rise thye are likely to
experience. If they get a Nat Gas or Propane line then a single unit
might make sense (if they went with one of the largest available).

Nevertheless, to appropriately assess the situation you need some more
facts: Specifically you need to know what they believe their maximum
simultaneous hot water usage might be. For example, if they are
expecting to be able to take 2 showers, run the DW, and wash clothes
simulataneously then you get a very different answer than if they are
willing to live with never using more than 1 source of hot water at a
time. Figure on 2.5 GPM per outlet while in use and you'll quickly
come to a total GPM that the tankless unit needs to produce.

From there you can simply assess the various manufacturers specs for an
approx. 70-80 degree temp rise (depending on water inlet temp and how
hot they like their water) and see whether the unit can provide the
necessary volume of hot water.

Doug