If you're going to be serving food and drink, I would get the extra chairs. It's hard to juggle a plate and glass and try to eat while standing. If all 11 are there at the same time, and eight find a seat, will the other three be standing, awkwardly trying to eat while everone else is comfortably seated?
Does the count of 11 include you, your mother and her BFF? If so, then it might be just you three standing unless one of you takes one of the seats, leaving a guest standing which would not be so nice. But honestly, as hostess, sometimes I do want to sit, even for just a couple of minutes, so it would be much nicer to have enough chairs.
I'd get the chairs and leave them folded and to the side and pull them open as needed.
If it's just drink with no food, then chairs might not be as important.