Jamieson, Fausset and Brown on Acts 1:15-26

15-26. in those days--of expectant prayer, and probably towards the close of them, when the nature of their future work began more clearly to dawn upon them, and the Holy Ghost, already "breathed" on the Eleven (Joh 20:22), was stirring in Peter, who was to be the leading spirit of the infant community (Mt 16:19).

the number ... about an hundred and twenty--Many, therefore, of the "five hundred brethren" who saw their risen Lord "at once" (1Co 15:6), must have remained in Galilee.