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What you can do is instead of having a line where guests can write how many will attend just have something like this:
___ Yes, I/we will be there!
___ No, I/we will not be able to attend.

Proper etiquette is to address the invitation to who is invited. Example: Miss Jane Doe and guest
Or Miss Jane Doe

If only one person`s name is on the invitation, it should be just that one person is invited. I hope this helps!