How many knights could be seated around King Arthur's Round Table?

Confusion which exists over the the number of knights admitted to the Round Table. This varies dramatically, depending one which literary source one examines:

13 in the "Didot-Perceval" (c.1225)
50 in Robert De Boron's "Merlin" (c.1195)
60 in Jean D'Ouremeuse's "Ly Myreur des Histors" (c.1350)
130 in the English ballad, "The Legend of King Arthur" (16th century)
140 in Hartmann Von Aue's "Erec" and "Iwein" (late 12th century)
150 in the Vulgate "Lancelot" (c.1220)
250 in the Vulgate "Merlin" (c.1225)
1,600 in Layamon's "Brut" (late 12th century)

http://www.earlybritishkingdoms.com/arthur/knights.html

The round table was massive!

The general concensus from varying sources is 150. This is based on the number of knights: 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 Siège Perilous ('danger-seat'), were left for those who might prove worthy.

https://www.arthurian-legend.com/knights-round-table/

It depends on the source.

From medieval sources:
According to Robert de Boron’s Merlin, the order of the Round Table contained 50 knights.

But in the Prose Perceval, also attributed to Robert do Boron (probably wrongly according to most modern scholars), it consisted of 13 knights, more closely corresponding to the participants at the Last Supper of Jesus, according to most artists.

The Knights of the Round Table at the time of the Grail quest in this romance may have been: Perceval, Kay, Bedwyr, Gawain, Gaheriet (approximately Malory's Gareth), Guerrehet (approximately Malory’s Gaheris), Mordret, Yvain son of Urien, Yvain White-hands, Lancelot of the Lake, Sagremor, Dodiniel, and Erec son of Lac. Or it may be a coincidence that exactly 13 well-known Arthurian knights appear in this work, matching the number of seats.

In the Prose Lancelot the number of knights of the Round Table is 150 and this number appears again and again in subsequent works. But according to Layamon’s Brut sixteen hundred and more could sit at the Round Table. As usual, one doesn’t find complete consistancy in legendary material.

http://www.lib.rochester.edu/camelot/FAQ-Arthur.htm
http://www.bartleby.com/211/1210.html