King Arthur did have a round table.
One myth says that Merlin built it for him, so that every man was equal when they sat it, none more important than the other.
But a more prominant one states that King Leodegranz, Guinevere's father, bestowed it upon Arthur when the two married, sort of like a part of her dowry.
It was one of a kind, in both myths. It seated an extremely large amount of Knights (100 - 200), and was never reproduced.