The average water conserving shower heads that are common today have a maximum flow of 2.5 GPM. When you mix the hot and cold water at the shower to get a comfortable temperature, (about 103-104 degrees) you are taking 2 gallons per minute of hot water. (2 gallons hot, ½ gallon cold). With this in mind, a basic tankless unit will supply enough hot water to run 2 showers at the same time. Other appliances vary in how much they take from the water heater.