Click http://www.britannia.com/history/arthur/knights.ht ml
and a link will take you to a web-page that explains - about halfway down - how the number of Knights varies depending on which literary source you look at. It's far from always being 12...one even goes as high as 1,600! If you scroll down the same page, you will find the names of the 25 which, it is claimed, are actually inscribed on the famous Round Table on display in Winchester.