Depending on the tradition one reads, besides Arthur there were between 12 knights and over 1,600. The most commonly accepted number, however, is the 25 knights shown on the Winchester Round Table.
The tradition of the Round Table may have started with the Celtic custom of allied warriors meeting in a circle so no one took precedence over another. In the written stories, it was first mentioned in Wace's "Roman de Brut" in 1155. Christians seized upon this idea of a round table, arguing that St. Luke recorded that the Last Supper was eaten at a similar table. This is probably the origin of the tradition of 12 knights plus King Arthur.
Some researchers point out that the Winchester Round Table, a physical table reputed to be authentic, only held 25 knights and measures over 18 feet across. A round table larger than this would be both unwieldy and impractical.