Here are some "short answers" but not the only answers.
First, to size the plumbing within the building you will probably be required to use the demand fixture unit method described in the plumbing code. In the U.S. this is most likely to be the Uniform Plumbing Code (UPC). This method tends to be quite conservative with respect to domestic demands.
BUT, you may have other demands to consider. For example, you may have irrigation demands, internal fire sprinkler system demands, boiler feedwater demands and possibly others. Each of these requirements may be governed by other Codes and authorities.
For public water mains and hydrants outside the building fire flows are usually the governing factor in main sizing. A typical design criteria for public main would be Maximum Day Demand plus Required Fireflow at 20 psi residual ( MDD+FF@20psig ) pressure.
For internal sprinklers local and national Fire Codes will govern and dictate flow rates.
good luck