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; Numbers 3:40; Numbers 3:43; Numbers 26:53) and in Revelation 3:4; Revelation 11:13.

Together (επ το αυτο). The word "gathered" is not in the Greek here, but it does occur in Matthew 22:34 and that is undoubtedly the idea in Luke 17:35 as in Acts 2:1; Acts 2:44; Acts 2:47; 1. Corinthians 11:20; 1. Corinthians 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.