code: def number_of_nodes(self, t=None):
          
          if t is None:
              return len(self._node)
          else:
              nds = sum([1 for n in self.degree(t=t).values() if n > 0])
              return nds
docs: Return the number of nodes in the t snpashot of a dynamic graph.
  
          Parameters
          ----------
          t : snapshot id (default=None)
                 If None return the number of nodes in the flattened graph.
  
  
          Returns
          -------
          nnodes : int
              The number of nodes in the graph.
  
          See Also
          --------
          order  which is identical
  
          Examples
          --------
          >>> G = dn.DynGraph()   # or DiGraph, MultiGraph, MultiDiGraph, etc
          >>> G.add_path([0,1,2], t=0)
          >>> G.number_of_nodes(0)
          3