The number of tankless water heaters you need to install depends chiefly on your household water consumption rates. It as well depends on the tankless water heater model you settle for. If you get a tankless water heater with a higher flow rate, it will be enough to handle all your household hot water applications. It’s up to you to decide what system to add to your household, depending on your core personal preferences. You can settle for tankless water heaters intended to supply hot water to one or two showers simultaneously. If the consumption rates in your household are a bit high, you should consider getting several units, though.