Who succeeded Judas Iscariot as an apostle?

Answer 1

Matthias. See Acts 1 for details of his appointment.
Matthias, was chosen to succeed Judas, for we read from Luke account in Acts of the Apostles, chapter 1, verses 20 to 26:

Peter continued, "This was written in the book of Psalms, where it says, 'Let his home become desolate, with no one living in it.' It also says, 'Let someone else take his position.'

"So now we must choose a replacement for Judas from among the men who were with us the entire time we were traveling with the Lord Jesus, from the time he was baptized by John until the day he was taken from us. Whoever is chosen will join us as a witness of Jesus' resurrection."

So they nominated two men: Joseph called Barsabbas (also known as Justus) and Matthias.

Then they all prayed, "O Lord, you know every heart. Show us which of these men you have chosen

as an apostle to replace Judas in this ministry, for he has deserted us and gone where he belongs."

Then they cast lots, and Matthias was selected to become an apostle with the other eleven.