12?

As the table is just a myth, who knows? Though in order to keep it sensible not too many. Lots of knights would need a bloody big circle...

Either 25 or 150, depending on how you look at it: http://www.arthurian-legend.com/more-about/more-about-arthur-3.php There is a big Round Table hanging on the wall of Winchester Castle, which names 25 shields. Sir Thomas Malory in Le Morte d'Arthur identifies Camelot as the English town of Winchester (disputed by William Caxton, Malory's own publisher, who asserts that Camelot was in Wales) and there has been a long and popular association between King Arthur and the Knights of the Round Table and the actual Winchester Round Table, but its origin has been dated to around 1270, the start of the reign of King Edward I - like the knights, well after Arthur's time. In literature, the Round Table varies in size according to which author is decribing it. The consensus is that it seated 150, with one chair - the Siège Perilous ('danger-seat') - which no-one could occupy safely except for the true Grail-Knight: the knight destined to achieve the Holy Grail, or Sangreal, a symbolism sometimes linked to the Last Supper, which had one place for Judas of ill-omen. The Grail-Knight - it was said that the Siège Perilous was reserved for Sir Perceval, then later, Sir Galahad - was required to be a hero with the purest heart, who was chaste and a virgin without sins (which disqualified Sir Lancelot from the start). The breakdown of the seating arrangements is this: King Laudegraunce brought 100 when he gave the table to King Arthur, Merlin filled up 28 of the vacant seats, and King Arthur elected Sir Gawain and Sir Tor - the remaining 20 seats, including the danger-seat, were left for those who might prove worthy.