I think all that totally depends on the dynamics of the party. Sometimes people just gather in the kitchen regardless of available seating. Some people tend to find a spot to sit in and never move. Some people only sit if others are. Some people like to talk to the same person or people all party long so sitting or standing, they never move much.
Now, in a party of say, 25 people, there's almost surely going to be a combination of these and other kinds of people and the seating might not be that important. Not to mention that seating 25 people in most homes might be difficult.
But here we have a group of 11. It's small enough that everyone could end up in the living room at the same time. If so, inevitably the majority will end up sitting and those who can't find a seat will be extremely few and it might be awkward.
Sitting on the floor might be an option, but it's not always ideal, regardless of age. It might depend on how people are dressed for example.
So again, so much about this depends on who will be there, how they are likely to congregate (which is usually unpredictable), what food is being served, how casual or formal the party is, etc.
I think getting a few folding chairs and having them available just in case is the way to go.