la Table Roonde, Tavola Ritonda
Our first extant knowledge of this table appears to come from Wace's "Roman De Brut". Chrétien mentions the table at least, but seemingly as an already established tradition.
Turning to Malory and his immediate sources, we find that Uther Pendragon gives this table to King Leodegrance, who in turn gave it, with 100 knights, to Arthur on the occasion of Arthur's marriage to Guenevere.
For practical purposes, the full complement would have been 149. Malory remarks in his colophon that "when they were whole together there was ever an hundred and forty", possibly always allowing a few seats for worthy newcomers. At least one modern romancer has considered that Merlin and Queen Guenevere were also allowed to sit in council at the Round Table: conceivably there were 140 seats for knights, with an extra ten for King, Queen, and non-knightly counselors.
The Table itself seems to have been nonmagical, although it had symbolical significance. According to the Vulgate there were three great tables: the one at which Christ and His apostles ate the Last Supper, the one at which Joseph of Arimathea and his disciples sat when they came to Britain, and the Round Table. The roundness of the Table symbolizes the world. As is fitting to complete this symbol, the Knights of the Round Table come from all parts of Christendom and heathendom. Even baptism may not have been a prerequisite of membership; Sir Palomides seems to have been a companion of the Round Table before his baptism.
Listed Knights at the Round Table | The Legend of King Arthur
Sieges | The Legend of King Arthur
Evidence found of ancient tribute to King Arthur's Round Table | Daily Mail Online, 28 August 2006
Time Team solves mystery of the round table (at Buckingham Palace) | The Telegraph, 29 August 2006
Round Table (Tournament) | Wikipedia