Address the envelope properly.
If a single person is being invited with a guest, the words "and Guest" should appear on the outside envelope. If a couple is being invited with their children, include the words "and family" on the outside envelope. If only some children are invited (such as older children), be sure to include their first names on outside envelope (Mr. and Mrs. John Smith and Sam). The lack of "and Guest" and "and Family" implies that no extra guests are included.
Include an inner envelope.
While some will consider it a waste of paper and money, a formal invitation commonly includes another envelope once you open the outside envelope (common with wedding invitations). If you include an inner envelope in your invitation set, you should print/write the name of each person being invited (first names only) on the front of this envelope.
Tell them exactly how many are invited.
The best option, to avoid any misunderstanding, is to include the number invited directly on the invitation or RSVP card. Simply have your invitations/cards printed with a fill-in-the-blank blurb (allowing you to write in the number attending):
We have reserved ____ seats in your honor.