Two people who come as a pair are one invitation, not two. Sam & Lee get one link, one message, and one answer that counts two.
Add the second name to a person's party when you add them, and the invitation addresses both of them. The default headcount is however many names are on it, so a yes from a couple counts two without them doing anything.
Plus-ones are a separate setting. Allow extra guests on an event and a guest can answer for more people than you listed — Yes, four of us — up to the number you allowed. Go over it and we record the number they actually gave and tell you, rather than quietly clipping their family off your count.
Kids can be counted separately. Turn on the kids question and a guest who is coming is asked how many children are coming with them, which is the number you need when you are working out food and chairs rather than seats.
Your people list holds the household, so the next event you make already knows that Sam comes with Lee.