Acts 1:15

The sin of Judas was not only his shame and ruin, but it made a vacancy in the college of the apostles. They were ordained twelve, with an eye to the twelve tribes of Israel, descended from the twelve patriarchs; they were the twelve stars that make up the church's crown (Rev 12:1), and for them twelve thrones were designated, Mat 19:28. Now being twelve when they were learners, if they were but eleven when they were to be teachers, it would occasion every one to enquire what had become of the twelfth, and so revive the remembrance of the scandal of their society; and therefore care was taken, before the descent of the Spirit, to fill up the vacancy, of the doing of which we now have an account, our Lord Jesus, probably, having given directions about it, among other things which he spoke pertaining to the kingdom of God. Observe, I. The persons concerned in this affair. 1. The house consisted of about a hundred and twenty. This was the number of the names, that is, the persons; some think the men only, distinguished from the women. Dr.

— Matthew Henry's Concise Commentary (public domain)

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 (Mat 16:19). the number . . . about an hundred and twenty--Many, therefore, of the "five hundred brethren" who saw their risen Lord "at once" (Co1 15:6), must have remained in Galilee.

— Jamieson-Fausset-Brown Commentary (public domain)