It should not be 11. Twelve is the correct number. The promise for some present to sit on the twelve thrones, as marked by the Hebrew "amen, amen" which translated into "certainly!", did not refer to Judas. More than the Twelve Apostles were present at this gathering. We know this from Acts 1:15ff. When Peter stands before the 120, he states they must replace Judas with a disciple who had been present for all of Jesus' earthly ministry-from the baptism by John until the Ascension (Acts 1:21, 22).
Of the 120, two met those criteria: Joseph Barsabbas (called Justus) and Matthias (vs. 23). When the lots were cast, it fell upon Matthias as the Lord's choice to take the place of Judas as one of the Twelve (vs. 26). Assuming Judas' place as one of the Twelve, he would then be granted one of the thrones to sit upon mentioned in Matthew 19:28.