I have seen a fair amount of code like in the (modified) OP in DB retrievals. The query returns a list, but the business rules say there can be only one element. And then, of course, something changed for 'just this one case' to a list with more than one item. (Yeah, I said a fair amount of times. It's almost like they ... nm)

So rather than then create a constant, I would (in clean code method) create a method to give a name or make clear what the conditional is intended to detect (and encapsulate how it detects it):

    public int getMoneyByPersons(){
      if(isSingleDepartmentHead()){ 
        // TODO - return money for one person
      } else {
        // TODO - calculate and return money for people.
      }
    }
 
    public boolean isSingleDepartmentHead() {
       return persons.size() == 1;
    }