Wow I can't believe that people actually crossed out the number and changed it! It's good to know that crazy things like that actually happen!

we had a similar rsvp
M__________________________ for them to write their own names
Then Accept with pleasure, decline with regret
Then a spot for No. attending ___
We were very open, and children and guests were allowed, so we didn't feel the need to point anything out to our guests.

I hear the thing to do with people who add more than invited, is to call them and say, "Unfortunatly due to x and x reason, the invitation was only addressed to Mr & Mrs. .. and not plus x y z"  I'm going to get my family members to do it (based on who's guest it is) bc I'm not standing for ANY of that nonsense .. our guestlist is outlandish as it is.

We also had our response cards say "We have reserved __ seats in your honor."  We only had one person try to add someone anyway.

We also wrote a small number on the back of every card in case someone didn't write in their name, which ended up being a great idea!

Mine are just number attending and then a spot for names and one of my guests just added their 14 year old daughter (whose name is not on the envelope inside or out).  problem is, its one of my professors and i don't know how to nicely say no!