I have a 100K Net boiler with 3 zones and a 4th zone for my 40 gallon indirect. We take 2 showers at once, then 2 back to back. Never a shortage and the home temp stays pretty constant. A big tub will whipe it out, and if you have issues with running out you can install a priority to keep the heat off as long as it is making hot water.
Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced