We know after the account in John 6 that the twelve remained. We also know in this account that Jesus said one was a devil and this was Judas hence the eleven.
Luke records that Satan entered into Judas.
At this point he was not saved as Satan cannot enter what the Holy Spirit occupies.
Was Judas saved later. I don't know.
We know after the account in John 6 that the twelve remained.
Yes, we do know the twelve remained and that Jesus spoke to them. We also know from the previous verse that many no longer followed Him. It doesn’t say that all except the twelve left Him. That may have happened.
I don’t remember where I heard this, but someone mentioned that the two disciples mentioned in Luke 24 had been with Jesus from the beginning of His public ministry up to His crucifixion. Maybe not, but that’s what prompted my question.
I agree with what you said about Judas. We do not know. Only God knows what was in his heart at the moment he breathed his last.