CE Douglas points out (Last Word in Prophecy, 134-5) that Greeks as much as Hebrews were obsessed with sevens:

“There were seven wonders of the world, seven sages, seven metals, seven tones and so on. Thebes, the city of the Phoenician Cadmus who taught the Greeks their letters, had seven gates. Even the grouping of cities in sevens has a respectable ancestry in the Aegean Sea. Thera . . . was organized by one of Cadmus’s companions as a lead of seven poleis.”

In Revelation, all the sevens have a corresponding “eighth.” Seven messages go to seven cities, but there are to candidates for the eighth city - Babylon and Jerusalem. There are seven lamps burning before the throne, but the Lamb is the lamp of the bridal city (21:23). There are seven stars in the hand of Jesus, but there are references to another star, the morning star (2:28; 22:16). The seven seals are connected with the “seal” of the living God (7:2), and the seven trumpets are completed by an eighth, a voice like a trumpet that is the voice of Jesus (1:10; 4:1).

In each case, the eighth plays the role of the key notes: “the group contains seven units, one of which (properly understood) controls or fulfills the function of the whole. It is, to take the musical parallel, the keynote of its octave - the source of the ‘harmony’ (i.e. tuning) of the whole.”