15 In those days Peter stood up among the believers (a group numbering about a hundred and twenty)

Read Acts 1:15 Using Other Translations

And in those days Peter stood up in the midst of the disciples, and said, (the number of names together were about an hundred and twenty,)
In those days Peter stood up among the brothers (the company of persons was in all about 120) and said,
During this time, when about 120 believers were together in one place, Peter stood up and addressed them.