If I remember correctly, I think that St. John Chrysostom said in a homily that the thrones are not literal but metaphoric for the honors they will receive. I could be wrong though. Assuming that there are literal thrones though--and I know this is controversial--I think that Judas is the one who will sit on the twelfth seat. I do not think that he was fully lost. He repented and felt enough guilt to give up the money, cry and kill himself. The only problem is that he let despair get to him and as a result killed himself when he should have thrown himself at the feet of his Saviour and prayed. I think that Judas is probably in Heaven right now crying, hugging Jesus being comforted, reconciling.