Where does Scripture say that the number dropped so that just the 11 were “really followers”?
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.