Acts 1:12-16
Then they returned to Jerusalem from the mount called Olivet, which is near Jerusalem, a Sabbath day’s journey away. 13 When they had entered the city, they went up to the upper room where they were staying; that is, Peter and John and James and Andrew, Philip and Thomas, Bartholomew and Matthew, James the son of Alphaeus, and Simon the Zealot, and Judas the son of James. 14 These all with one mind were continually devoting themselves to prayer, along with the women, and Mary the mother of Jesus, and with His brothers.
15 At this time Peter stood up in the midst of the brethren (a gathering of about one hundred and twenty persons was there together), and said, 16 “Brethren, the Scripture had to be fulfilled, which the Holy Spirit foretold by the mouth of David concerning Judas, who became a guide to those who arrested Jesus.
Sometime after Jesus performed the miracle of fish on the Sea of Galilee in front of the seven disciples, and after Jesus encouraged Peter to continue to follow Him in discipleship (Jn. 21:1-23), He gathered all eleven of His disciples together on the Mount of Olives where He spoke His last words to them and then ascended to heaven (Acts 1:4-9).
The disciples then, according to Jesus command, immediately went to Jerusalem (a Sabbath day’s journey) to the upper room, where they were staying. We don’t know what kind of arrangements they had made with the owner; whether they purchased the room, or were renting it, or he was just letting them stay there. But we know that they ate the last supper there; and it was also where Jesus appeared to them after His resurrection. Now they would stay there as a place of safety, and to wait and pray until the day of Pentecost (vv. 4-5).
It appears from the text that the upper room was rather large; for one hundred and twenty people were gathered there, waiting for the coming Holy Spirit.
As for Peter, he had apparently felt the strong call of God to lead this group of disciples. And here it seems that he was led by God to give his first sermon, mainly having to do with electing another man to replace Judas. And as it happened by lot, Matthias was the one who was divinely chosen (verse 26).