Properly speaking, "number of guests" shouldn't be included, because as we stated in an earlier post, only those whose name is on the envelopes is invited. The M________ is for Ms. Jane Stack or Mr. and Mrs. Robert Smith, etc. Everyone attending, who was invited, should be named on the reply card.
If sending a regular card, a self-addressed stamped envelope should be included. If sending out a postcard, don't forget the stamp!