In most cases, you will need one Water Management Plan (WMP) per property/site.
If your campus has several buildings, you will need only one WMP provided they are all managed by the same personnel. One WMP could accommodate even a university campus or large office park with more than 100 buildings.
Buildings located on different sites/properties should have separate WMPs even if the buildings are similar in size and type and located in the same city. An exception could be buildings located on properties that are adjacent or across the street, provided they are managed by the same personnel and your flow diagrams make sense.