The senior members of Invisible College on Earth-40, all of whom a knighted peers of the realm in the Order of Avalon. These knights are entirely unknown either inside or outside the organisation and only ever referred to by their codenames, the names of Arthur’s companion knights. The most senior knight is only ever referred to by their seat at the Table; the Siege Perilous.
Members of the Round Table are bound by geasa and are likely to be have been exposed to the breath of the dragon.
The number of knights is neither publicised, nor static, and it may be that no one can say for sure just how many there are at any given time. Notably, there is never a knight codenamed, “Arthur,” who is, after all, the Once and Future King. Having a second Arthur at the table on the old king’s return was deemed too awkward to contemplate.