Were there more believers in Jerusalem than the 120 in the upper room?
No. According to the context, after three years of the Lord's earthly ministry there were only 120 Jewish believers gathered in the upper room in Acts chapter 1. In the area of Jerusalem and Judea, there must not have been any more than these 120, though a few may have been in Galilee.
Les explains that even during Christ's earthly ministry, the number of believers in Jerusalem was very small. He points to Acts chapter 1, where 120 Jewish believers were gathered in the upper room. Les says this is shocking after three years of the Lord's own ministry. He allows that there may have been a few believers up in Galilee, but in the Jerusalem area there must not have been any more than those 120. This small remnant contrasts with the millions brought in during the Church Age through the preaching of the Gospel of Grace.