A.T. Robertson Commentary Acts 1:15

"And in these days Peter stood up in the midst of the brethren, and said (and there was a multitude of persons [gathered] together, about a hundred and twenty)," — Acts 1:15 (ASV)

Brethren (αδελφων). Codex Bezae has "disciples."

Multitude of persons (οχλος ονοματων). Literally, multitude of names. This Hebraistic use of ονομα=person occurs in the LXX (Numbers 1:2; Numbers 18:20; 3:40,43; Numbers 26:53) and in Re 3:4; 11:13.

Together (επ το αυτο). The word "gathered" is not in the Greek here, but it does occur in Mt 22:34 and that is undoubtedly the idea in Lu 17:35 as in Ac 2:1,44,47; 1 Corinthians 11:20; 14:23. So also here. They were in the same place (το αυτο).

About a hundred and twenty (ως εκατον εικοσ). A crowd for "the upper room." No special significance in the number 120, just the number there.