I was very surprised when I researched for the etiquette of giving admission tickets to a wedding, and I couldn't find anything.
In my Country, it's normal etiquette to include as many tickets as the people you intend to invite. For example, if it's a family of 5, but you are just inviting the parents, then you include 2 tickets. Or if it's a single friend but he/she can bring somebody, then you include 2 tickets on his/her invitation.
What is the etiquette in the United States?