Water Heater Size Requirements

Does anyone have a reference for the number of water heaters/size of them based on the number of bedrooms? I think 3 bath is one 50 gallon, 3 1/2 is two 40 gallon, etc…

Here is the reference that I use…

http://rightspec.bradfordwhite.com/Sizing/Residential.aspx%between%

http://www.uniongas.com/home/ngforhome/appliances/choosewaterheater.asp

http://www.energyright.com/waterheat/choosing.htm

http://www.hometips.com/help/wat1.html