About 120 people, according to Acts 1:15, where Luke reports that in the days between Jesus' ascension and Pentecost "the number of names together was about a hundred and twenty" — including the apostles, the women, Mary the mother of Jesus, and his brothers.
Acts 2:1–4 describes the Spirit's arrival on the day of Pentecost without giving a fresh headcount, so the 120 figure from the preceding chapter is what the text supplies for those gathered in the upper room.
Two related numbers sometimes appear in this discussion, worth distinguishing from the upper-room count:
- Acts 1:15 — ~120 disciples in Jerusalem → the upper room on Pentecost.
- 1 Corinthians 15:6 — Jesus appeared to "above five hundred brethren at once" after his resurrection, per Paul's later summary. This is a different event (a post-resurrection appearance, likely in Galilee), not the Pentecost gathering.
So for the upper room on the day the Holy Spirit came: roughly 120.